How does the mobile syncing work?

Really looking for a system where I can install the app on my parents' iPhones and it backs up their photos to my server without them having to even know about the app. They won't open it, ever.

Nextcloud fails at that.

It's pretty easy to set it up to upload automatically, if that's the question. No need to launch it. I've only got my server on my home network so I can't sync while away, and I occasionally check to make sure it's working - no problems at all yet, a few months in.