I think the problem lies in the way the iphone started out as an ipod + phone. So it’s still using the ipod synch system (which works passably for syncing music, videos, and photos from your computer to a device that can play them but not make them), with an ever-higher pile of add ons and extensions for other types of data piled on top. By now it’s an unmanagable rube goldbergian mess held together with string, duct tape, and chewing gum. Some data types appear to have fallen through the cracks and never had an extension created for them (local copies of sent emails, for example, are gone forever when you backup and restore. Hope you didn’t care about any of that data of yours.) Layer over it all with a byzantine set of rules about transferring your media from one device to another (in an attempt to cut down on piracy to placate their music store partners) which have explanatory dialogs that are confusing enough to have been written by Microsoft, and you have the shambling horror that is Itunes.
This is why there’s no simple way to back up everything on your phone and restore it to a different phone, creating an exact copy of your old phone’s data. Aside from those data types that just plain don’t transfer (sent emails, the cards you’ve registered in Apple Pay, etc), you have this arbitrary and stupid split of user data into “stuff that syncs,” and “stuff that backs up” plus “photos you’ve taken” which appear to obey yet another set of rules, and it’s a confusing mess.
