How do you stop syncing across devices but still enable iCloud?

All devices using that same iCloud account, and with things like iCloud photos turned in, will sync all photos and videos. Same thing for notes and another sync’d data enabled on each device.

That is the whole point of iCloud - it doesn’t store content independently of devices. It holds all data from all devices sync’ing new data to it it to enable all that data to be sync’d across all devices using that iCloud account and sharing data with that iCloud account.

So you cannot have separate devices sync’ing data to iCloud and not sync’ing all that data across all devices.