Photos

iCloud Photos keeps one library in sync across every device signed into your Apple Account, which means a change on one device happens everywhere.

Settings > Apps > Photos > Sync this iPhone

Full resolution photos stay in iCloud and your iPhone keeps smaller versions, downloading the original only when you need it.

Settings > Apps > Photos > Optimise iPhone Storage

Deleted photos stay in a Recently Deleted album for 30 days before they are removed permanently, and they still count against your storage until then.

Photos > Albums > Utilities > Recently Deleted

Hidden photos are moved out of the main library into a Hidden album, which can itself be hidden from the albums list and locked behind Face ID.

Settings > Apps > Photos > Show Hidden Album

A Shared Album lets chosen people view, comment on and optionally add photos, without using your iCloud storage allowance.

Settings > Apps > Photos > Shared Albums