I've had WeChat on my phone since a vacation to Beijing last year, when friends there essentially ordered me to download it. More than 760 million people use it regularly worldwide; it's basically how people in China communicate now. It's actually a lot of trouble not to use WeChat when you're there, and socially weird, like refusing to wear shoes.

In China, 90 percent of internet users connect online through a mobile device, and those people on average spend more than a third of their internet time in WeChat. It's fundamentally a messaging app, but it also serves many of the functions of PayPal, Yelp, Facebook, Uber, Amazon, Expedia, Slack, Spotify, Tinder, and more. People use WeChat to pay rent, locate parking, invest, make a doctor's appointment, find a one-night stand, donate to charity. The police in Shenzhen pay rewards through WeChat to people who rat out traffic violators—through WeChat.

Illustrator: Steph Davidson

It's nothing special to look at, as far as smartphone apps go. The first screen that opens is the chat stream; a menu at the bottom gets you to other areas, like a WeChat wallet and a "moments" stream for Facebook-like posts. Companies, media outlets, celebrities, and brands also open "official accounts" that you can follow to get news and promotions. The design stands out only for its relative simplicity and calm; the online mainstream in China is overpopulated with weird click-bait and manic GIFs.

Zhang Xiaolong, WeChat's creator and something of a cult figure in China, has called WeChat a lifestyle. I rolled my eyes when I first heard that. Then I went back to Beijing in April.

My colleague Lulu Chen, who covers WeChat's parent, Tencent, has sent me the phone numbers of some potential contacts—but why call when WeChat is so much easier? I use the chat function to set up meetings during my visit. One of my contacts mentions a WeChat convention the day after I arrive, and so, on a Sunday afternoon, I show up at the Design Service Center, an industrial-chic space in the historic city center. The crowd is mostly young, a mix of Chinese and expatriate, and the mood is festive. Free wine stands three bottles deep on the bar.

I drift by company displays and find myself at the table for Yoli, a business that offers a sort of speed dating for English learners: 15-minute on-demand tutoring sessions with native speakers through WeChat. Two sheets of paper taped to the table each bear a pixelated QR code: Scan one to become a teacher, scan the other to become a student.

The Chinese term for this ritual, sao yi sao, quickly becomes familiar. Everyone and almost everything on WeChat has a QR code, and sao yi sao-ing with your phone is both constant and strangely satisfying. James, a tanned American with unruly blond hair who mans the Yoli table, is here to host a workshop called "How We Built a WeChat App & Recovered Our Development Costs Within 24hrs." He scans my code, which gives him my WeChat profile and also generates the equivalent of a friend request; I accept, and we agree to meet during the week, skipping right over the old-fashioned niceties of last names and business cards.

The presentations are about to start, and jet lag is kicking in. I hurry to the coffee counter for an iced Americano. There's a QR code in a plastic photo frame. The woman ahead of me is scanning it. I try it, and … WeChat fail. I've entered a credit card into WeChat, but it won't work, and my WeChat wallet is empty. I feel distinctly self-conscious fumbling around for yuan. I've been in WeChat-era China one day, and already cash money feels embarrassing.

Shake, which connects the user with a random person to message with.

On Monday, I take the subway to meet Zhu Xiaoxiao, who's built a WeChat-based fitness business. On the train, I notice a woman moving methodically down the car, stopping to talk to the other passengers. Is she begging? Testifying? Only when she stops before the woman next to me do I get it: She's asking for QR scans, trying to get followers for a WeChat official account.

Zhu is an open-faced, bulked-up 25-year-old in a gray T-shirt, blue shorts, and red sneakers. He left China for school in England a skinny kid and returned in 2012 a fitness buff with the germ of a business plan—to make and sell protein powder. He and a friend developed a formula, set up manufacturing and a website, and began marketing online. In late 2013, Zhu started looking for investors, and the next February he got 2 million yuan—roughly $300,000—from a seed fund in Beijing. At the urging of his investors, he stopped selling the protein powder and refocused on building a following of health enthusiasts, opening a WeChat official account that pushed articles on exercise and diet and lots of pictures of six-pack abs. The company, FitTime, quickly racked up 400,000 followers and an additional 9.8 million yuan in funding, and launched a standalone app.

As WeChat boomed, Zhu developed a fitness camp on WeChat, an alternative to expensive personal training in a physical gym for people already on WeChat all the time. Sign up, and you get grouped into a chat with 15 people of similar height and weight and a personal trainer who's there to motivate you (by message and emoji) to stick to the diet and video workout plans. FitTime charges 1,000 yuan for 28 days, and more than 5,000 people have signed up for at least one month.

Stories of sudden success on WeChat abound these days, and Xi Jiutian's is another. She's wearing oversize nerd-cool glasses and bright-red lipstick when we meet for lunch on Tuesday at Cafe Groove. The place looks like something out of my Brooklyn neighborhood, the mismatched chairs, the random shelves of books, even the prices—$10-plus for an avocado salad. This is all familiar—until I go to pay with WeChat, and my credit card is rejected again. I'm definitely losing some face here.

Xi was an interaction designer at Microsoft in Beijing before getting laid off. She tried designing a smartwatch, then consulting for startups. She also began writing on Zhihu, a site similar to Quora, about makeup and skin care. In early 2015 she opened Hibetterme—as in, "Hi, better me"—a WeChat account devoted to the same topics. After a couple of months, her WeChat fans began urging her to sell beauty products. Setting up a shop on WeChat's platform took her a couple of days. Xi, like Zhu, had an easy time finding funding when she began looking last fall. She'd been at it about a week when a friend of a friend put her in touch through WeChat with Eric Tong of Pros & Partners Capital in Shanghai. After they'd messaged on WeChat for about 15 minutes (a lot of their discussion was about tattoos), Tong told her to stop her search and committed 4 million yuan.

Xi introduces us on WeChat, and Tong responds instantly. But when I try to set up a phone call, he ignores me. People seem to talk on the phone less than they used to—though they're happy to leave each other WeChat audio messages. I ask, by chat, how Hibetterme fits with what he looks for in an investment. In a flurry of abbreviations, he says he's looking for professionally generated content across platforms like WeChat. It's an investment theme that's very, very hot, thanks to the Papi impact. Papi is Papi Jiang, known for her speed-talking comedic video monologues. In April, she auctioned off the first advertising spot to appear in one of her videos for 22 million yuan. Um, bubble? Tong's fund stands at about 200 million yuan now. He expects to have 600 million by the end of the year.

The basic message stream.

Even those who aren't directly selling things or running official accounts on WeChat use it constantly for work. A friend who runs restaurants in Beijing operates his entire operation, almost everything except eating and drinking, on WeChat. He trades dish ideas and discusses kitchen operations with the chefs in one group, while his accountant keeps him informed of payments on another. There's even a group devoted to flower care at one of the restaurants. (WeChat introduced a formal enterprise version in April.) Yoli, the tutoring company, takes the all-WeChat model to extremes. James, the American I met on Sunday—his last name, I finally find out, is LaLonde; he's from Texas—moved to Beijing to found a gaming company in 2011. He decided last August to combine his interest in language learning with an experiment in creating a business run entirely on WeChat. It made sense; he rarely left the app as it was. He's met Luke Priddy, one of his two co-founders, only twice in person. Priddy lives in New York and coordinates the growing cadre of teachers. The average wait time for a tutoring session is 20 seconds. The tag line for teachers is "teach on the beach"; Priddy once conducted a tutoring session while floating in a pool.

On Wednesday, I need to get to Shanghai for a day of meetings and can't decide whether to fly or take the train. Buying train tickets with an app may not sound revolutionary, but in China, I promise you, it is. The intricacies of buying tickets used to occupy whole sections of guidebooks and require feverish strategizing before holidays. Opening WeChat, I check the train schedules and get to the point of booking an overnight train—but then decide to fly. I can't quite shake my fear of the Chinese train system.

WeChat has made Beijing a very different place from the city I lived in from 2006 to 2009. There's so much less standing in line and waiting, particularly at the bank. Cash used to be king. I paid my rent in cash, my bills, every restaurant and shop. Now people shoot money around on their phones (not all on WeChat, of course, but a lot of it).

There's also a lot less getting lost. Taking a taxi in China used to require getting the driver to call your destination to verify exactly where you were going. On this trip, everyone I visit drops a map into a message, with the location pinned, and I show that to the driver. The one time I get turned around, walking to an interview, I open real-time location in the WeChat conversation I'm having with my host. She finds me on the map and guides me.

Nobody's too cool to use WeChat, or too uncool. It's how entire families keep in touch. A tech executive told me his mother, at 80-plus, uses it for everything; a marketing entrepreneur said his computer-illiterate parents and his daughters, ages 3 and 5, use it.

The WeChat wallet.

By Thursday morning, I've decided something important: I don't like my QR code. The code WeChat randomly generated for me looks like a piece of candy in a blue wrapper. When I click on "Change Style" in my profile, it goes from bad to worse—a piece of toast? A cat? A pink car? Finally, some algorithm spits out a green, leaf-shaped design. I'll take it.

I've also given up on using my credit card. It's "accepted" by WeChat, and I've set up a PIN and all that, but I guess WeChat can't change the fact that few local businesses take international cards. WeChat has given life in China a smoothness, a quality of efficiency I never could have imagined. But for a foreigner like me, at least, it's still a work in progress.

I message a Chinese friend who's in the U.S. on a fellowship and ask for a loan. Within minutes, he's sent me two hong bao, or red envelopes—a play on the red envelopes traditionally used to give gifts of money. They arrive as chat messages that say, "Good fortune and good luck! You've received a red envelope." Once I click on them, I have 200 yuan in my WeChat wallet.

Typically, you hand out red envelopes of cash to younger relatives and friends during the Lunar New Year—to couples getting married, for children's birthdays. Now hong bao are used … I don't want to say willy-nilly, but sometimes just for fun.

It's hard to tell what's great strategy and what's luck in WeChat's success, but this hong bao system is genius. The company wasn't first with electronic hong bao; that would be Alipay, the payment platform from Alibaba. But when WeChat introduced its own system just before the Chinese New Year in 2014, it added a gaming element. When you send money to a group of people, one lucky winner within the group gets a bigger windfall than the rest, while a few get nothing at all. People love the element of chance, apparently, because users of WeChat's wallet jumped by 100 million in a month. The figure is now 300 million. For Chinese New Year 2016, 516 million people delivered 32 billion red envelopes.

Midmorning, I go to the Global Mobile Internet Conference in the China National Convention Center. Hundreds of speakers, 20 summits, and a music festival—it's China's South by Southwest, or trying to be. I'm exhausted from running from floor to floor to catch sessions. I stop at a cafe on the second floor to get coffee, my new WeChat riches teed up.

They don't take WeChat. At a tech conference.

The next day, I return to the conference to talk to E Hao, co-CEO of the group that organizes it. I'm accosted in the elevator by a young woman who sees that I'm foreign, explains that her company organizes exchanges with foreign companies, and demands to scan my WeChat QR code. "Nice to meet you!" she sings, striding off without ever telling me her name or asking for mine.

E Hao is hoarse after a late night at the event's opening gala at the Olympic Bird's Nest stadium. His heavy metal band, CXO, newly formed with various fellow executives, performed for the first time. He shows me his WeChat message stream: 3,015 unread messages. He says he's been relying on hong bao to thank and motivate his overworked employees through the long days running up to the event, sending out 1,000 yuan at a time. He sends me 100 yuan to demonstrate. I'm not sure about the etiquette. Is this for demonstration only? Should I send it back? I do, eventually.

When I get back to New York, I join a FitTime WeChat boot camp. The rest of my group seems to be Chinese students studying in the U.S., including the trainer, who's in Iowa. First, there's the horror of taking a selfie in spandex and sending it to a stranger, then the awkwardness of photographing every meal, with one hand held in a fist beside the plate for perspective on serving size. If I'm lucky, the trainer sends me a thumbs-up emoji in response. She frequently has to remind me of the rules, though: No kimchi, for example—too much salt, leads to bloating. The whole thing is vaguely humiliating. On the other hand, I've lost a few pounds, and I now know the characters for chia seeds in Chinese. And I'm on WeChat all day long.
With Lulu Chen