iCloud doesn’t do backups for Mac so much as it mirrors (syncs) everything when that’s enabled. Delete a file in one spot and it’s gone everywhere, etc.
There’s no means to exclude parts from the mirroring, short of storing the files somewhere not mirrored—in another macOS login not enabled, for instance, or in the Sharing folder. (In /Users/Shared — Share files with others who use your Mac - Apple Support. I don’t believe that gets synced with the per-user-enabled iCloud files and documents sharing, but don’t have a way to check that as I don’t use iCloud desktop and documents sharing.)
Here’s an Apple writeup on the controls (the direct link to this Apple article is offline right now) to the desktop and documents sharing:
https://web.archive.org/web/20210521125323/https://support.apple.com/en-us/HT206985
If you are bandwidth-limited, consider switching from iCloud desktop and documents sharing to local Time Machine backups. (This does mean you need to be more explicit about maintaining what you are sharing via iCloud or otherwise, if you are working with more than one Mac.)
Local Time Machine gets you local backups and with some depth. Time Machine can exclude files and folders from backups too, though that’s usually less of a bandwidth issue with most local backups.
A four to six terabyte external hard disk drive is typical for Time Mache backups, given current external hard disk drive prices, and given internal Mac storage of, say, two terabytes or less, and with decent depth of backups.
If you have more than one local Mac, a Network Attached Storage (NAS) server box can be used ro pool backups across all Macs, including Macs connected by Wi-Fi. The NAS must support Time Machine for this to work.