How do you backup MacBook Pro to iCloud without using external drive

You cannot back up a Mac to iCloud. You would need to back up to an external drive. Many people use Time Machine to backup. https://support.apple.com/en-us/HT201250. You generally want to get a drive that is at least 1.5x the size of the items you are backing up.

You can sync your photos, desktop and documents, and iCloud Drive with iCloud but that's not a true backup. Rather, it keeps your stuff in sync across devices.

In this case, my problem is that after upgrading to Monterrey just today, my computer (Macbook Pro M1) is not reading any external drive. They're connected directly to the USB port, not via a hub. And I need to downgrade to Big Sur. -That'll be another question, how to do that :o/