macOS Big Sur-How to Downgrade to Catalina

I walk through the downgrade process, highlighting new problems along the way that are specific to Big Sur.
00:00 Introduction
00:11 Prepare a drive for backup
01:03 Partition external USB drive
03:00 Time Machine problem
03:57 Backup without buying software
05:02 Disable System Integrity Protection
06:39 Startup Security-If you have a T2 chip
08:44 Backing up with cp
10:36 Backup drag and drop apps
11:30 Backing up with ditto
12:41 Find Catalina download
14:00 Create external USB installer
17:31 Before you downgrade do this
18:11 Erase Big Sur drive with Big Sur Recovery mode
23:00 If you forgot to erase with Big Sur Recovery mode
Errata: After changing Startup Security settings (for T2 chip owners) you must Shut Down, not Restart. Then power on and continue.
How to create a bootable installer for macOS
support.apple.com/en-ca/HT201372
How to get old versions of macOS
support.apple.com/en-ca/HT211683