Where can I find comprehensive explanation of Apple-syncing and backup & how to delete out-of-date backup files from iCloud?

SYNC and BACKUP are two very different things.

Apple does not provide a clear path to doing one, or the other. It is very UNclear.

Backup CREATES a new file. Sync UPDATES an existing file. It sounds like Apple sync is a bit weird in that if you do something to EITHER file, it changes BOTH files. That sounds a little hard to control and scary with a lot of potential to mess things up.

I use a great sync (and backup) software that syncs my current data drive TO my backup data drive (but not the other way around - doing something to my backup data drive will NOT effect my current active data drive (i.e. if it should get deleted, it doesn't delete all my active data, too). The sync maintains an exact duplicate of my current active data drive on my backup data drive.

Right now, I have reservations about using iCloud at all. What with making everything so proprietary and so incompatible with everything else in the world, Apple has rendered iCloud pretty much useless for anything EXCEPT backing up an iPhone. Since two iCloud accounts can't communicate with each other, or even be used alternately by your iPhone, I certainly wouldn't BUY apps or media via an iPhone with a locked-in single iCloud account. Nor would I store my photos in iCloud - I would get them into a normal image format and get them on normal, accessible storage ASAP.