If you have the offload feature turned on, downloading a new app or an OS update can cause least recently used apps to have their app bundle deleted while their

If you have the offload feature turned on, downloading a new app or an OS update can cause least recently used apps to have their app bundle deleted while their Home Screen icons remain, such that they'll be downloaded again when you next launch them. I think the app sandbox files are kept on the phone, so you're still logged in. It's a way to cope with not having a big enough flash disk. OS updates are bigger than apps so the feature unblocks some users from upgrading. To your last question: Contacts don't take much space, so probably never. Photos and videos, if you use iCloud sync, you can choose to hold lower or original quality on your device but either way iCloud is the authoritative storage point. I don't think they get offloaded like apps when you need more space but they're big enough for that to be realistic. Conversely I think when your local storage is full, new photos taken elsewhere won't sync down.