If I can only restore a back-up created with Catalina, what happens when Apple sends an upgrade to my iMac and it now runs with a new OS ? How do I restore those backed up files if I need access to them with the new OS?

Personally, I would set aside the Time Machine backup Drive that was used on Catalina for safe keeping.

Get a New External Drive and start a New Time Machine backup on the new drive.

One can still access the Catalina Backups if some files are on it the same way as when using Catalina

You are welcome and thank you too.

Do come back to the ASC Forums, ( Apple Support Communities ) in the future, if / or when additional question may arise