How can I uninstall Big Sur and return to Catalina?

You can't uninstall an OS Upgrade. Everything old was replaced.

You can erase the drive and restore from the backup you made prior to installing Big Sur.

Or, you can erase the drive and reinstall the old OS from a bootable USB installer of the OS you want to use, then migrate from your backup.

If you didn't make a backup, it is a little more difficult to get some of your data back.