I think you shall only buy flights from US to China direct - no BA, LH etc. there was a change implemented just about 10 days ago that if you transit in 3rd country you need to have negative test results as well from 3rd country and certified by Chinese embassy. So this renders 3rd party transit not possible anymore...

also, whatever your landing point is in China, you quarantine there for 14 days so it is needless to book a transit in China - it is not allowed anyways... why you can not book transit in US - that I do not know...