How to disable auto-exposure adjusting for my webcam?
I use Webcam Settings which works fine for me on Big Sur (although not specifically updated for it yet), but I use an external webcam.
I'm not sure if it would work for you or not. It does provide some controls for the built-in webcam on my M1 MacBook Air.
It sells for $8 on the US version of the Mac App Store, and you could always request a refund if it did not work (although this is a clear example of a case where it would be helpful if Apple allowed for demo apps on the Mac App Store, that does not seem like something they are ever going to do).