I find it very interesting that we apparently are several that seem to have experienced the same chain of events: updating to Big Sur - camera not working (at all).
I feel like I’ve tried everything. Talked to Apple support, very friendly, but couldn’t solve it and ended up saying it was an unrelated hardware failure. I ended up accepting that, since downgrading didn’t solve the problem. I am now on Big Sur 11.1, and the problem is still there. But again, VERY interesting that more people seem to experience the same thing.
I have also checked the system report, done the diagnostics when rebooting, reset SMC and PRAM, all to no avail. The system doesn’t detect a working camera.
I realise this post isn’t much help, but thought I’d clarify my status.
Cheers, Jacob