[newscracker]

Whenever an app asks for access to Photos to save an image or to read a photo, it gets access to the entire camera roll (all photos). So it can certainly upload your entire photo library someplace if it wants to, subject to foreground usage time, network speed and background activity (limited by iOS).

Compared to that workflow, if you open the stock/official photos app and use the share sheet to share one or more photos with a specific app, then it would get only the selected photo(s).