If something like the B-21 was used against China's mainland proper I think China would reply with an ICBM launch with either conventional or nuclear weapons depending on the damage done. At best it can be used tactically and that is assuming China wouldn't escalate. The whole bomber program is only good at dealing with 2nd tier nations without a viable nuclear deterrent. Assumptions it would be usable against other nuclear weapon states are a mirage I think.
B-21s would likely be based on Hawaii or Whiteman Air Force Base in the Continental USA
Depending on the damage done, yes, China could respond with conventional or tactical nukes against either of these locations
But the end of the day, I think what China wants is the same respect accorded to the Russian core interests
The US military is knows that if the USA and Russia get into a direct military conflict, it's all too likely to work up the nuclear escalation ladder. We can see this playing out with Ukraine as an example, where the US has avoided direct military action
So you can substitute Russia/Ukraine for China/Taiwan