When her middle school closed in March, Garvey Mortley stopped going to lacrosse practice and playing drums in the school band. With so much time at home, she leaned into another interest: Roblox, an online gaming site and app with Lego-like characters and millions of virtual worlds to explore.

Sprawled on the floor of her living room in Bethesda, Md., Garvey began logging more hours in the online universe, building virtual houses, adopting digital pets and racing other players in obstacle courses. She said she now plays Roblox on her laptop for up to five hours a day while chatting with friends on her phone, up from an hour or two before the pandemic.

“It’s like my main passion,” said Garvey, 12. “It’s pretty diverse, and you can meet people around the world.”

The coronavirus has created some pandemic winners as people shop in droves on Amazon, buy Peloton bikes to exercise at home and head to drive-in movies. For children, there are pandemic victors, too — and chief among them is 14-year-old Roblox, which was already popular but has become wildly so since people have been urged to stay at home.

Since February, the number of active players on Roblox has jumped about 35 percent to reach 164 million in July, according to RTrack, a site that tracks Roblox data. About three quarters of American children ages 9 to 12 are now on the platform, according to Roblox. And players spent 3 billion hours on the site and app in July, twice as much as they did in February, the company said.

Inside Roblox, which is free to play, gamers create an avatar. They can play the site’s millions of games, bringing their character into environments ranging from tropical islands to haunted castles and bustling towns. Their avatars can engage in first-person combat, decipher puzzles or participate in egg hunts while chatting and interacting with other players. Gamers pay real money — often $5 or $10 at a time — to become premium members and to purchase an in-game currency called Robux, which lets them buy clothing, weapons and even hot air balloons for their characters.

“At a time like this, where people are housebound, being able to escape into the digital world and have these kinds of fun, imaginative experiences with a friend, is very, very relevant,” said Craig Donato, Roblox’s chief business officer.

For players, Roblox has an element of never-ending discovery because independent developers create more than 20 million new games a year for the platform. And for those developers — who are mostly teenagers, college students and young adults operating solo or with a small team — the recent spike in Roblox’s popularity means boom times for them, too, since they get a cut of the money that users are spending on their games.

One beneficiary is Anne Shoemaker, 20, who said she moved from Palm Coast, Fla., to Silicon Valley two years ago with $100 in her pocket to live near other developers as she coded games for Roblox. For a while, she said, she didn’t earn enough from the platform to justify turning it into a full-time job.

But after the pandemic hit, Ms. Shoemaker saw a surge of interest in her two Roblox games, Mermaid Life, a fashion-focused role-playing game, and My Droplets, a pet simulation game. Players have paid for extra content in those games, and for the hats she made for users’ avatars that she sells across the site.

Ms. Shoemaker said she has now made about $500,000 through Roblox, most of that since March. Before the pandemic, she could afford to pay just one or two people to help her. Now her game studio, Fullflower Studio, employs 14 contractors and she is plotting new games.

“It feels incredible,” Ms. Shoemaker said. “People used to tell my mom, ‘Stop letting her play this video game; it’s not going to get her anywhere.’ And it’s getting me somewhere.”

Roblox said it has more than 2 million developers, 345,000 of whom make money and who split their profits 50-50 with the company. Dozens of the top developers make millions of dollars, the company said, and top games in the past have generated an average of $2 million to $3 million a year.

Revenue from Roblox’s mobile app, which most players use, totaled $493 million in the first half of the year, up from $228 million in the same period a year ago, according to SensorTower, an analytics firm. Roblox declined to disclose its financials, except to say it was cash-flow positive.

Roblox, which is based in San Mateo, Calif., was founded in 2006 by Erik Cassel and David Baszucki, who were engineers and entrepreneurs. Mr. Baszucki is the chief executive; Mr. Cassel died of cancer in 2013. The start-up has raised $335 million from investors including Meritech Capital Partners and the Chinese internet giant Tencent. In February, when it raised fresh financing from investors including Andreessen Horowitz, it was valued at $4 billion.

In Roblox’s early years, its growth was slow. But activity began picking up in 2015 and 2016 as technological tweaks made it easier to play on phones and Microsoft’s Xbox. Roblox has now become at least as popular as Minecraft, one of its main competitors, said Craig Sherman, a venture capitalist at Meritech.

Even with its recent surge in activity, Mr. Sherman said, Roblox is “on the cusp” of more growth. He said the platform has become a foundation for people to start businesses on, similar to YouTube.

“Roblox is becoming this generation’s version of going to the mall or downtown with your friends, and Covid probably helps accelerate that,” he said.

Some children who grew up on Roblox have never left. Alex Balfanz, 21, started coding games on Roblox when he was 9. In 2017, as a high school senior in Orlando, Fla., he released a Roblox game called Jailbreak, in which people’s avatars are prisoners attempting to escape from jail or police officers trying to keep them imprisoned.

Mr. Balfanz said he watched in astonishment as Jailbreak tallied more than 70,000 players at once on its first day. The game has now been played more than 4 billion times and makes several million dollars a year, which Mr. Balfanz said he splits with his business partner.

In the pandemic, Jailbreak has reached even greater heights. Mr. Balfanz said that on the best days, the game has 80,000 to 90,000 people playing at once, compared with 40,000 to 60,000 before.

“It was a pleasant surprise, something I hadn’t really thought of happening in the midst of all the unfortunate circumstances brought by the pandemic,” he said.

With his earnings, Mr. Balfanz, now a senior at Duke University, said he has taken some vacations,paid his college tuition (Duke’s tuition is nearly $60,000 a year) and bought a Tesla. He has attained celebrity status on Roblox, and said he is swarmed by star-struck players whenever he logs on.

But the dangers of the offline world have sometimes seeped into Roblox, including reports of extremist groups attempting to recruit children and the appearance of sexually explicit content in games.

Mr. Donato said that safety was Roblox’s top priority. The company reviews game content, has extensive parental controls and filters profanity and personally identifiable information out of chats, he said.

“We see all sorts of major real-life events, from Covid-19 to racial discrimination somehow impacting our platform,” he said.

Back in Bethesda, Garvey recently used Roblox to educate others on racism. When players participated in virtual Black Lives Matter protests in the game this summer, she noticed some were darkening their avatar’s skin color, ostensibly a statement of solidarity with Black people. But to Garvey, who is Black, it seemed more like “virtual blackface.”

So Garvey made a YouTube video explaining the history of blackface in the United States and urged Roblox users to dress their avatars in Black Lives Matter T-shirts instead of changing their skin color.

“I took an educational route,” Garvey said. “I was trying to just seem a little helpful, not just angry at everyone.”

Garvey’s mother, Amber Coleman-Mortley, said she was proud of how Garvey reacted. She said she views Roblox as an ideal place for education, especially with in-person learning and socializing curtailed.

“Play is the way that the human mind learns best,” she said.