If new photo is added to library and app is not running in foreground or was not opened after the new photo was added but the app is having full access to gallery, can it access, read the new photo - If the app is not specifically a cloud syncing app, can it have this attached function, suppose it is a game app or beauty camera app?
If your app has been granted Full Access to the Photo Library by the user then it may use PhotoKit whenever the app gets runtime.
I'm not entirely sure if this is what your question is though, could you describe what you're trying to accomplish with more depth?
Suppose a new photo is clicked by the user but an app wasn’t opened actively after the new addition, can apps read it or they need to be opened at least once in the foreground after the change?