The Chinese wouldn’t have received much stealth tech from the shot down F-117. Knowing the materials used is one thing. Being able to manufacture them is another. Not to mention, the F-117 was 1970′s stealth tech. We have advanced greatly since then; though they’ve probably already stolen that tech through their hacking.
As for the F-117 shoot down and the embassy bombing, it is my understanding that the Chinese supposedly helped the Serbians with bistatic RADAR technology to shoot down the F-117. One of the many methods stealth aircraft use is to be stealthy is to redirect the RADAR energy away from the aircraft in such a way that it does not reflect back toward the emitter. However, by placing the emitters and receivers at different locations you could triangulate a stealth aircraft’s position; At least one of F-117 era stealth tech. But it depends on the locations of the emitters and receivers wrt the aircraft.
We made this easy for them to figure out, because the USAF became so comfortable with their supremacy, they didn’t bother changing the routes the F-117 was flying, so it’s flight path became predictable. So that, combined with bistatic RADAR tech, made the shoot down possible. It was my understanding that we then “accidentally” bombed the Chinese Embassy as a result of their aid to Serbia with this technology.
As for the J-20, their new “stealth” fighter, we’re still not sure if it is a prototype or just a demonstrator. The next few months will reveal that answer. However, it isn’t as stealthy as the F-22, but that is most likely by design/due to requirements.