How to downgrade macOS Big Sur to macOS Mojave (or Catalina, High Sierra, etc.) From backup to restore without spending money, to restore.

I walk through the downgrade process, highlighting new problems along the way that are specific to Big Sur.

00:00 Introduction and what you need
02:38 If you have TouchID, delete your fingerprint
03:00 Turn off Find My Mac and sign out of iCloud
03:47 Export data out of Apple Photos, Notes, etc.
04:23 Prepare our backup drive
06:40 How to open and setup Terminal for backup
10:54 Backup our home directory with cp
12:55 Backup/update our backup with ditto
13:57 Download the macOS installer app of your choice
15:14 How to create a macOS bootable USB installer
16:09 Handling the "too old to be opened" error message
16:47 Create our macOS USB installer on our backup drive
20:23 Restart in Big Sur's Recovery Mode
21:25 If you have a T2 chip we must change Startup Security settings
22:40 Erase our drive with Disk Utility, but in a special way
24:11 T2 Chip owners shut down, others restart with option key
25:04 Erase our drive with Disk Utility again
25:46 How to erase the drive with Terminal if Disk Utility failed
26:57 Run the installer
27:01 Setup
27:47 Caution about copying files with Finder
28:20 Restoring from our backup, manually
29:03 Restoring items from the hidden Library folder
31:39 Repair home directory permissions to avoid problems
33:16 Install software or download it and install
33:36 Conclusion

How to get old versions of macOS
https://support.apple.com/en-ca/HT211683

How to create a bootable installer for macOS
https://support.apple.com/en-ca/HT201372