Why is my iPhone still showing full storage after deletion?

If you delete photos and videos, they might just go into the Recently Deleted album. You won't get the storage back until they are permanently deleted from that album.

If you are using iCloud Photos, you could turn on the "Optimize iPhone Storage" feature, which would allow the phone to store low-quality versions of some photos and videos on your phone (to save space) while preserving originals in iCloud. Note that you have to let the phone manage the process - manually deleting a photo will be taken as a request to delete it not only from the phone, but from iCloud and all other devices synced to it.