iCloud storage

Note that there is a difference between

• Device storage – the storage built into your iPhone (which can't be increased), and
• iCloud storage – storage on Apple's servers, used for backups and synchronization

What's the difference between device storage and iCloud storage? - Apple Support

With iCloud, you get 5 GB of free iCloud storage, but you can rent more. The free 5 GB is plenty for calendars and contacts, but it's very easy to run over that limit if you have a lot of photos and videos and are synchronizing them through iCloud Photos; or including them in iCloud backups.

iCloud+ - Apple

Aside: If you have lots of photos and videos and you aren't saving them to iCloud in either way, then you need to be backing them up in some other way. It's too easy to lose them all if they are stored only in a single phone that could be lost, or stolen, or that could fail, or that you could be forced to reset due to a security lockout.

When you delete photos and videos, you don't immediately get the storage back.

The photos and videos go into a Recenty Deleted album – like the Trash on Macs, or Recycle Bin on Windows – to give you a chance to get them back if you deleted them by mistake.

After 30 days, the system automatically does a permanent deletion of the photos and videos to recover the space. You can go into the Recently Deleted album before then, and permanently delete stuff yourself, if you want to get space back faster than that. But be warned: once you permanently delete a photo or video, there is no changing your mind. You can't get it back.