When backing up your phone to iCloud and then factory resetting your phone, I always assumed that iCloud is just a mirror image of your phone so once your phone is wiped, how come iCloud doesn’t also wipe itself? An example being photos, delete one from your phone and it auto deletes from iCloud.
When you turn on your new phone for the first time and connect it to iCloud, how come iCloud doesn’t look at the new phone and say “Oh, this phone is empty so I’d better mirror image it and delete my content?”
I realise iCloud doesn’t wipe itself, so have I misunderstood how iCloud works?
iCloud does two different things: sync content and back up iPhones/iPads. The example you gave is referring to syncing (iCloud Photos); not a backup (iCloud Backup).
When you use "iCloud Photos" and delete a photo, the app tells iCloud to delete it. When you wipe a phone, you're just erasing an encryption key to render the phone's encrypted storage unreadable. It doesn't go through and delete every photo, nor does it tell iCloud to delete everything.
For the things you have set to sync (photos, messages, notes, contacts, etc.), the content in iCloud that doesn't exist on your iPhone/iPad/Mac will get downloaded. Content on your iPhone/iPad/Mac that doesn't exist in iCloud will get uploaded. The only time content gets deleted from iCloud (and other devices) is when you specifically delete them. Remember, these sync between all your Apple devices if you have them turned on.
Now an "iCloud Backup" is more or less a mirror image of the iPhone/iPad that was backed up. It's essentially the same as the backup file you make with iTunes/Finder, but stored in iCloud. The backups will contain the all stuff that's not syncing to iCloud, including third party app data. For example, if you're not using iCloud Photos, the photos will get included in the backup itself. Deleting a photo on your iPhone would not delete it from the backup. (To get the photo back, you would need to wipe the device and restore from the iPhone backup stored in iCloud...just as you would if the iPhone backup was stored on your computer.)
This article should explain it more:
iCloud Backup helps keep your data safe by making a copy of the information on your iPhone and iPad that isn't already synced to iCloud.
support.apple.com
