Impossible to know accurately.
if the US were to fly B2s over to China, would they be able to find them and shoot them down?
There's a lot of factors that go into this question that have nothing to do with the quality of Chinese IADS, but even so, lol, lmao, absolutely not. The most widely used strategic SAM in the Chinese arsenal is the HQ-9B. The HQ-9 itself is heavily based on the Russian S-300. The HQ-9B is heavily modified, with (we think) mid course uplink and (we think) terminal radar and infrared guidance. The HQ-9P is an variant made specifically for Pakistan, and we saw how well it worked there. The HQ-9 would very likely be absolutely shit on in a shooting war with the US.
The HQ-15 is basically an indigenous variant of the S-300PMU-2, which is to say it, too, would be shit on mercilessly.
The HQ-16 is an upgraded version of the Russian Buk. Interestingly enough it was actually co-developed with Russian assistance. It likely would perform well against non stealth jets and subsonic cruise missiles like the tomahawk. Again, against B-2s or F-35s it would be shit on mercilessly.
The HQ-19 is basically the Chinese version of THAAD or Aegis Ashore. It is primarily an ABM with alleged exo-atmospheric intercept capabilities. It's been successfully tested against conventional ballistic missiles but not in exo conditions or against HGVs yet. Western analysis generally points towards it being pretty capable, though likely still in LRIP. Unknown if it has hit IOC yet.
The HQ-22 is a medium-long range mobile system likely derived from the S-300 family with significant modifications inspired by the US Patriot. It is believed to primarily be for mobile medium range operational level defense against a wide variety of targets.
The problem with the majority of these is that they have never seen combat. The ones that have seen combat, like the export models in Pakistan, leave a lot to be desired. Moreover, Chinese early warning and detection radar seems to lag significantly compared to the Russians or the West, and the Chinese have shown absolutely no novel capabilities to counter US stealth aircraft. Overall, against US air power? I'd rate their capabilities extremely low, and extremely vulnerable to US SEAD/DEAD. Against US missiles? I would rate them likely highly effective, although this is mainly because the primary US missile used is still the fucking tomahawk, which is a fat, slow bitch. Not to say it's bad, just long in the tooth. Additionally, as I've said in this thread and others, measures are always cheaper than countermeasures, and the US has a truly comical number of tomahawks. So many fucking tomahawks. Destroyers pack like 96 of them, then you have the 4 converted Ohios which carry over a hundred. In a shooting war, China would very quickly be forced to decide which missiles it intercepted.
