You deleted hundreds of photos and the storage bar barely moved. It's almost always one of these five reasons.
Deleted photos stay in Photos → Utilities → Recently Deleted for 30 days and still count. Empty it (Select → Delete All) and the space comes back.
With iCloud Photos and Optimize iPhone Storage, your iPhone only kept small versions of older photos. Deleting them frees much less than their full size suggests. Large videos you recorded recently still free the most.
Photos and videos you sent or received in Messages are stored separately from your library. Check Settings → General → iPhone Storage → Messages → Review Large Attachments.
Caches, logs and temporary files show up as System Data. iOS clears them by itself when space runs low; a restart often shrinks it. No app can clear it for you.
The storage screen can take a few minutes to recalculate. Close Settings, wait, and open it again.
Videos are usually where the space is. Lumaroll has a Large Videos category, sorted so the biggest files come first, and shows how much space your decisions free.