A woman who had a negative test (well published that the best tests are~80% accurate and the Chinese tests bought by EU were much less accurate) may have left virus in a lift (more likely if she coughed or sneezed, many people cough for other reasons and might not realise it's anything) that one neighbour picked up using the lift (fomite risk is low, but going into an enclosed space that someone has breathed and coughed virus into, potentially minutes later, very possible; even more likely is that they were directly near each other and simply forgot). People then transmit covid-19 as normal in parties, at work, etc. We can't necessarily believe the Chinese government's explanations of events as fact, because they don't declare asymptomatic cases and are known to have hidden their symptomatic stats too, but the case is plausible. I don't know what's confusing or upsetting you about the facts presented?