iCloud
Dear Apple, I want to ask, why some of my photos and videos couldn't backup to iCloud?
iPhone 11, iOS 16
If you have iCloud Photos turned on they they do not back up to iCloud they are already there.
If you do not use iCloud Photos and want the phone's photos backed up make sure they are not sync'd from another device like your computer (they will not backup either), and that you have enough storage in your iCloud plan.