Earlier quoted context omitted.
I back them up on my Mac to BackBlaze and TimeMachine . They are just local files at that point.
Do you see all the photos in the photos app (and hence iCloud Photos) in iCloud Drive? Near as I can tell, they're separate and this is by design. This impacts pictures or videos taken on phones, iPad’s etc by default; as they go into iCloud Photos, not iCloud Drive.
You iPhone or iPad take photos and store them in a local cache that syncs to iCloud. It is separate from iCloud Drive but the storage comes from the same iCloud capacity that you have subscribed to.
On your Mac the files are stored in a library folder that can be on any local drive. Those files can be backed up like any other file. Any changes you make to those files, like in the Photos app, are synced to iCloud Photos.
The photos in the local library are accessible but they are not really meant to be accessed. The meta data about the photos is stored in a database, not the files. There are original files, modified files, and thumbnail files.
If you want to you can export some or all photos to separate files and do what you want with them.