The source of the Beijing mini-outbreak is unknown so far. One theory is imported salmon, which seems iffy. But they found the virus on a salmon cutting board at the market. Another theory is it came from somebody from Hubei (the Wuhan province). China has also been dealing with mini-outbreaks up at the border with Russia (near North Korea), which isn't too far from Beijing. And then Beijing is an international city and some flights are going in and out ...
Unlike the US, China did a pretty serious and effective 10-week lockdown. And then another 10 week gradual reopening. No half measures and then a rush to reopen. Full measures and then a steady slow careful reopening. It's been organized, coordinated and effective.
Outside of Hubei Province, where the virus commenced, China has reported 15K confirmed cases. Roughly the same as Japan and South Korea. The US has had more deaths than China has had cases. Must be that great Trump leadership.