Apple’s apps make it pretty clear when something is local and when something is in iCloud. Even for iCloud users, it’s usually an explicit choice per file or object. Photos is the notable exception.
I’m pretty sure there’s also a very-much-not-fine-print notice about iCloud Photos when you set up a device, and you can easily turn iCloud Photos off. The only part missing is that you can’t enable iCloud Photos but choose, per photo, whether to sync the photo.