Unable to update iPhone due to storage issue

I believe that when you delete photos, they go into a Recently Deleted album on your phone. That gives you a chance to recover your photos if you deleted them by mistake. The photos continue to take up space on your phone until they are permanently deleted, which happens

• After 30 days, or
• When you delete them from the Recently Deleted album

whichever comes first.

If you are using iCloud Photos, you may want to go into Settings and select Optimize iPhone Storage.

Manage your photo storage - Apple Support

This gives the phone permission to substitute low-resolution copies of photos for full-resolution copies when your phone's local device storage starts to get full. The full-resolution copies remain in iCloud (on Apple's servers), from where your iPhone can pull them down on an "as needed" basis (when you're connected to the Internet).