Shein, with a supply chain rooted in southern China’s Guangdong province, was until recently the top-ranked shopping app in the U.S., according to Sensor Tower. Arriving in the U.S. just in time for last year’s holiday shopping season, Temu raked in 13 million downloads in the fourth quarter, more than double that of Shein. Like Shein, Temu connects bargain hunters in the U.S. with China’s manufacturers, offering lower prices by cutting out the middleman. The companies use data to drive every decision.

Former ByteDance engineers say ByteDance is one of the most aggressive in executing a strategy known within the industry as “horse racing,” where multiple teams are assigned to build the same product or feature with slight variations. Once it becomes clear which version is performing better, the winning team is given more resources while the other versions are scrapped, these people say.

“People sometimes said the company was heartless because no one had complete control over a product design from start to finish,” Mr. Guo said.

ByteDance product managers and engineers also say the company has standardized protocols, systems and detailed metrics to assess what users like, which help it roll out new updates in a matter of days. TikTok’s signature single column scroll, for example, was a design it settled on after creating several user interfaces, including a two-column version similar to Instagram’s explore tab, current and former ByteDance employees say.

Behind the ruthless testing, reiterations and surveys for user feedback are long hours clocked by tech workers, who can get paid out several additional months of salary in bonuses based on their performance and output. Temu’s parent PDD, in particular, is known in the industry for demanding hours.

PDD has said its 2022 research and development investment jumped 15% from a year earlier, with much of it going to luring talent.

PDD’s quarterly sales and marketing expenses often exceeded its revenue between 2017 and 2020, when active buyers on Temu’s Chinese sister platform Pinduoduo more than tripled from 244.8 million buyers to 788.4 million. The company, which makes money primarily from advertising, first turned a profit after it went public in the second quarter in 2021.

Temu, like Pinduoduo and Shein, liberally doles out coupons and other incentives for downloading its app, hoping users will talk up the apps in their social networks. Marketing campaigns for Temu attempt to reach potential buyers in almost every channel, from Facebook banners to targeted emails. In February, Temu aired its first Super Bowl commercial.

Industry players say a push to dominate advertising is standard practice in China to win customers. “When Chinese companies see an opportunity, they are more willing to buy traffic at a much earlier stage and on a much bigger scale than their U.S. counterparts,” said Ivy Yang, a China tech analyst who formerly worked for e-commerce behemoth Alibaba Group Holding Ltd.

Chinese companies’ attempts to expand internationally haven’t always had runaway success. Alibaba’s international online marketplace, AliExpress, has been around for 13 years, but is far from a household name in the U.S. The first product ByteDance tried to push overseas, TopBuzz, a news aggregator, was a flop. The company later wound down the business.

For Temu, the current breakneck growth isn’t met with just plaudits; it has attracted consumer complaints about delayed shipping and poor product quality.

For now, Temu’s wired earbuds or dog leashes selling for less than $2 are having a moment capturing the attention of inflation-weary Americans.

“The 2008 financial crisis spurred Chinese manufacturers to sell on Amazon, ” said Mr. Fan, the venture-capital investor. “It’s Temu’s time to shine now.”