Most Apple apps wont open on Mac Big Sur
Ideally, you’d make a bootable copy of your primary drive, erase it, and reinstall macOS. The easiest, free option to do so is with SuperDuper but it’s not yet Big Sur compatible.
I think your next best bet is to make sure you have some kind of backup (Time Machine or otherwise so you can at least access your old files) and erase and reinstall macOS.
After you have your backup, hold down Command + R on startup to get into recovery mode. Open Disk Utility, select your primary disk (top-most level), erase it, and reinstall macOS. From there, you can access your backup and pull out the files you need.