I have Big Sur running, but most of my hardwares and drivers don't work... how do I get Mojave or Catalina Back????????

Make sure you're fully backed up, because this will erase your startup disk:

1. Fully shut down your Mac.
2. Power on your computer and immediately hold Shift + Option + Cmd + R. You'll notice startup takes longer than normal as macOS Recovery loads.
3. Once the macOS Utilities screen loads, choose Reinstall macOS (or Reinstall OS X) and click Continue.
4. Follow the prompts and choose your startup disk. Now click Install.
5. Walk through the on-screen instructions to complete the rest of the installation process.
6. Once the installation is complete, restore your files from the backup you created.

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Sorry you are having those problems. Just to let you know, generally speaking, after installing an OS upgrade and some updates, you will need to download and install new drivers for your printers, scanners, etc. That's just the way it is. Also, it is the responsibility of those vendors to provide drivers and software that is up to date and fully compliant with the new operating systems. Apple makes information available to all of them well in advance to do this. So, before going to the trouble of downgrading your OS, I would first install updates to your device drivers/software. That normally solves these problems.

https://www.imore.com/how-downgrade-macos