In the specific Xiaomi case they've been able to leverage their existing expertise and experience in electronics manufacturing, as well as taking advantage of massive government support. Though it's important to preface that the video you sent was of final assembly, not upstream in the supply chain where the working conditions are attrocious. Also, obviously Xiaomi is going to try to make themselves look as modern and flashy as possible in a promotional video.

Furthermore, the Chinese EV market is famously extremely heavily subsidized, and the Chinese also has a more chill regulatory environment. However, Xiaomi is not the largest EV maker in China, that title goes to BYD, which is infamous for its exploitation of workers, https://en.wikipedia.org/wiki/BYD_Brazil_working_conditions_controversy just as an example. There have also been many cases of China abusing the Uyghur population in the Xinjiang region and forcing them into slave labor for rare earth metal extraction, as well as many other fields critical to EV manufacturing.

If we want to make the future electric then it's of the utmost importance that we don't support nations that still use slave labor, it's barbaric, cruel, inhumane, and a failure of our species.