downgrade software

One can downgrade the OS only back to the version of macOS the computer hard when it was New. Example, if the computer came New with Big Sur macOS 11 - then Big Sur is the oldest version the computer can be downgraded to. The Apple Licence of Use Prohibits installing oder version of macOS for the reasons already outlined.

If the computer has run Catalina previously >>

This Method will WIPE ALL DATA including the Recovery Volume. This is not Recovery of Data - period.

To perform this action will require booting from a Bootable Installer. 

This will have to be performed from a Qualifying Computer to run the version of macOS to be made on the Bootable Installer. Example : Bootable Installer of Catalina would have to be done on a computer that Qualifies to run Catalina.

Once that is done read on for preparing the Destination computer 

>> Only works on Intel Based Apple Computers.

Extra Special Notation regarding the Touch ID equipped Apple Computer.

About Startup Security Utility and Must Enable from Recovery Mode the ability to boot from External Drive Before Attempting 

1 - Shutdown computer and disconnect all external drive Except the newly created Bootable Installer.

2- Restart and immediately hold the OPTION key until the Startup Manager appears and choose the USB Drive. 

3 - It will present options >> Disk Utilities >> View >> View ALL attached Drives. 

4 - Choose the Upper Most Drive ( not the volumes indented and list below ).

5 - The drive normally is called Apple Media or Apple SSD - that is the drive to Erase and format as APFS with the GUID Partition Map.

6 - Once that is done >> backup out of Disk Utilities and choose install macOS. 

7 - Follow the prompts and it may automatically reboot several time. 

8 - Upon a final reboot - Setup Assist will present with the newer version of macOS.

Welcome and Thank you too.

Do come back to the ASC Forums, in the future, if / or when additional question may arise

The Very Besting in the New Year !