Notes on backups you actually own.

Optimize iPhone Storage Explained: Where Are Your Original Photos?

Optimize iPhone Storage is on by default with iCloud Photos, and it quietly moves your original photos off the phone. What it does, how to tell, and what it means when you want a backup.

iPhone Storage Full? How to Free Up Space Without Deleting Photos

Your photos are almost certainly the problem, and deleting them is not the only fix. How to find what is really using space, move your library off the phone, and delete it safely.