Hello BillyBobGoat,
For most things iCloud is just a syncing service, meaning the items are still stored directly on your devices and synced with iCloud. So with Photos for example, if you're using the iCloud Photos service your photos are synced through iCloud. The purpose of this is if you're using multiple devices and want the same photos on all of them. So for example, if you have an iPhone and your Mac, when you use the iCloud Photos service your photos will be the same on all your devices. If you take a new picture on the iPhone, it will sync through iCloud and automatically show up on your Mac. If you delete a photo on your Mac, it will sync through iCloud and automatically delete that photo from your iPhone. Etc.
If you're using multiple devices and want your photo library's to remain independent on them, then you'll want to stop using the iCloud Photos service. On your Mac, you can turn it off in System Preferences > Apple ID > iCloud Photos. On an iOS device, in Settings > name at the top > iCloud > Photos.
Set up and use iCloud Photos
Now if you have the Desktop & Documents option enabled in iCloud, that is not a syncing service. That is storing anything you put on your Desktop or in your Documents folder directly in iCloud instead of directly on the computers hard drive. If you want all your files to be stored directly on the computer, then you'll want to turn that off in System Preferences > Apple ID > iCloud, then under the iCloud Drive section select Options and turn off Desktop & Documents Folders.
Add your Desktop and Documents files to iCloud Drive
Cheers!
