How does the mobile app sync work with Immich? My use-case is that I want to install the mobile app on the phone of my relatives (including iPhones), and it should keep syncing their pictures "forever" even though they never, ever open the app.

I tried Nextcloud, but the apps/server end up failing to sync after at most a couple month, and it's painful to recover from that. So it doesn't work for me.

I have been considering Immich and Ente, but I would love to know if somebody has experience with that.