Storage not updating after deleting photos and videos

When photos and videos are initially deleted, they are just moved to the recently deleted photos album in the Photos app. They will automatically be deleted from the album in 30 days. You can force an immediate deletion of the files by following this path,

iPhone > Photos > Utilities > Recently Deleted > tap View Album > tap Select in upper right corner of screen > tap 3 dots > tap Delete All

Wait for all files to delete and then restart your iPhone.

Until you delete them from recently deleted album in Utilities Collection they will still be on the iphone.

Longer term, Buy a monthly iCloud plan and use that to manage your space at least as big as your iPhone storage, eg 200gb for the price of a Starbucks a month.

Ensure you are syncing to your iCloud Photos (iPhone, Settings, [tap your name], iCloud, Photos, Sync this iPhone)

In the same settings ensure Optimise this iphone is turned on

If this is the first time you turned iCloud photos on leave it 2 days to sync on a good wifi.

 When you are sure that iCloud has all your images and movies you can backup the phone and erase it to factory status, then restore the backup.

The on-phone images will then be sync'd down from iCloud in the minimum size possible (aka thumbnails). You will also lose excess System data

Resources:

Set up and use iCloud Photos – Apple Support (UK)

How to back up your iPhone, iPad, and iPod touch - Apple Support

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support