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).

Same as @TJ Luoma suggested but different app has worked for me Exposure Adjustor you can always request a refund but it works for me.