2020 MacBook Pro screen flickering on and off

Another quick test that separates hardware from software: boot into macOS Recovery and just watch the screen for a few minutes. Nothing of yours loads there, so if it still flickers in Recovery it's the hardware, full stop. Then check whether it's still under AppleCare or any extended coverage before paying for a repair.

Plug it into an external monitor or TV. If the external screen is fine and only the built-in one flickers, it's almost certainly hardware, display or display cable. If both flicker, it's software or graphics. Worth noticing too whether it changes when you move the lid. Flickering at certain angles is the cable, and that's a repair job, not a settings fix. For the software side: boot into safe mode and see if it still happens, turn off auto brightness and True Tone to rule them out, and if it's the Intel model reset the SMC and NVRAM. The M1 one doesn't have either, a full shutdown does the same thing.