How come China caught up to US' 6th gen fighter development?

This video compares what we know of Chinese and US next generation fighter programs. What does this sudden appearance of Chinese planes mean for the US? How might these new Chinese planes be used? And how much might future US air supremacy be impacted? 00:00 Intro 01:37 State of US next gen programs 03:51 Demonstrator or prototype? 06:24 Just what has flown in China? 09:25 Roles, requirements and use 14:35 History of China’s next gen fighter 15:51 When might future fighters see service? 18:00 Cost issue