Mac mini Late 2020 won't boot but has yellow light indicator

A solid amber or yellow light (depending on what you want to call that color) means the device is in firmware recovery mode. Likely the firmware is corrupted, or damaged so instructions the computer needs to boot up properly are missing, or incorrect.

If you have access to another Mac you can try using Configurator 2 to revive or restore the device. Revive should not affect the onboard storage, and is generally safe to do without fear of data loss. Restoring the device will wipe the internal storage, so you will want to make sure you have a backup or have come to terms with data loss before you perform a restore.

If it’s stuck in recovery mode and Configurator 2 does not correct the issue, or you don’t have access to a second Mac, this might be a job for a repair shop depending on what the actual issue is, but it is likely to be a logic board level issue if revive/restore does not work. See Apple if you’re still under warranty or have AppleCare on the device.

If you do perform a restore and it fails, make note of the error you receive since it can help diagnose the issue.