Restore Catalina from big Sur

Not sure what the M1's have to do with anything. If you have an M1 Mac you can't install Catalina. If you have an Intel Mac…

Revert your Mac to a previous macOS version - Apple Support

I am confused. Do you have an Intel or Apple Silicon M1 processor? If it is an Apple Silicon M1 processor you cannot downgrade to Catalina. If it is Intel then what is the precise year of your MacBook Pro 13"? Apple menu then About this Mac. If it's an Intel based MacBook Pro 2018 or newer the process will vary.

Ethno-mc wrote:

hi - thanks for reply :-) my ignorance....it is a MacBook Pro (13 inch, 2016, two thunderbolt 3 ports) - and i want to go back to catalina -- i have made backup to time machine..not sure what to do next

How To Downgrade macOS Big Sur To Catalina Or Older ...

https://www.macworld.co.uk/how-to/downgrade-macos-3581872/

Follow the Link provided by @dialabrain called " Revert your Mac to a previous macOS version - Apple Support "- It will give all the details and how to do.