Yes, the app does iOS background sync. Many of us (I'm part of the Ente team) are avid users and have a huge photo libraries, and day-to-day, it works seamlessly. We don't need to reopen for it to sync etc. The only time one needs special care is when doing the initial import - at that time, we sometimes need to keep the app running in the foreground for the initial sync to complete.
As the other commenters are mentioning though, this is all black magic at the mercy of Apple. The way we've evolved with our code works now, but who knows what future updates to iOS bring. One thing we've observed that it takes sometimes like say seven days for Apple's on device ML to pick up that the user really wants to use the app, and convince the OS to allow the app to run in the background to sync. But again, this is not something we've needed to worry about as _users_ - we just use it normally as we'd use Apple Photos, and it just works after the initial sync completes.
As the other commenters are mentioning though, this is all black magic at the mercy of Apple. The way we've evolved with our code works now, but who knows what future updates to iOS bring. One thing we've observed that it takes sometimes like say seven days for Apple's on device ML to pick up that the user really wants to use the app, and convince the OS to allow the app to run in the background to sync. But again, this is not something we've needed to worry about as _users_ - we just use it normally as we'd use Apple Photos, and it just works after the initial sync completes.