Shein‘s quiet rise has reached a crescendo as the fast fashion e-commerce app takes the crown from Amazon as the most downloaded shopping app on iOS and Android in the United States, according to data from app tracking firm App Annie.

Its ascent is quiet because the startup, despite reportedly exceeding a $15 billion valuation, maintains an unusually low profile and doesn’t try to make itself known to the media. The app, dubbed the “TikTok for e-commerce” by China-focused internet analyst Matthew Brennan in this thorough piece on the startup, manufactures in China as many apparel retailers do.

The difference is Shein controls its own production chain, from design and prototype to procurement to manufacturing. Each step is highly digitized and integrated with another, which allows the company to churn out hundreds of new products tailored to different regions and user tastes at a daily rate. The strategy is not unlike TikTok matching content creators with users by using algorithms to understand their habits in real-time.

On May 11, Shein became the most installed shopping app on Android in the U.S., and six days later took the top spot on iOS as well. It briefly topped the Android chart in late April.

The origin of Shein, which was previously named “She Inside,” is little understood. On its official website, it describes itself as an “international B2C fast fashion e-commerce platform” founded in 2008. There is no mention of its founder and CEO Chris Xu. In a 2018 corporate blog posted on WeChat, it wrote that it was headquartered in Nanjing, an eastern Chinese city home known for its historical heritages and home to Chinese appliance giant Suning. It also opened offices in other major Chinese cities as well as the U.S., Belgium and the United Arab Emirates.

Shein’s low profile is perhaps expected in times of geopolitical tensions and heightened regulatory scrutiny over China-related tech companies around the world. Shein owns its sales channel and user data, which distinguishes it from the swathe of generic consumer brands relying on Amazon for customer acquisition without meaningful access to user data.

Last June, the Indian government banned Shein among 59 apps, including TikTok and WeChat, that it deemed “prejudicial to sovereignty and integrity of India, defence of India, security of state and public order.”

As of May 17, Shein was the top iOS shopping app in 54 countries and ranked top in the category on Android devices across 13 countries.

Shein has not announced who its investors are, but Chinese media reports have listed Capital Nuts, JAFCO Asia, Greenwoods Asset Management, IDG Capital, Sequoia Capital China, Tiger Global, and Xiaomi founder’s Shunwei Capital among its backers.

We’ve reached out to Shein for comments on the story. Sequoia Capital China confirmed it’s an investor in Shein.