Downgrade MAC Os from BigSur down to catalina on Macbook Pro early 2015

Hello RobPMac2015,

It sounds like you'd like to revert to a previous macOS version, and while it's recommend to stay up to date with your mac, the following article explains other macOS installation options available to you:

How to reinstall macOS

It will show the way to restore the original macOS that came with your mac as well as the latest macOS your mac is compatible with. Thanks for posting in the Apple Support Communities forum.

Enjoy your week!

You will most likely not have a choice of "best suited"; following the instructions in the posted links will either install the latest OS or a previous one (most likely the original OS).

If your Mac came with Catalina and you have a bootable external clone of that system with your files, then you can simply plug that in, erase your entire internal disk (after you make a current backup, just in case), and clone back the old Catalina.

Revert your Mac to a previous macOS version