iOS photo sync functionality question

I have seen on a couple occasions one of my family’s devices will stop syncing. Maybe it signed out and they didn’t notice or what. Because of that, sometimes they will miss uploading a handful of pictures/videos.

Let’s say there are many photos added, and even some removed from the phone that were already uploaded. What would the functionality be if “Upload the whole camera roll” was used? Would it only upload the new ones? Would it see photos on the iOS device were deleted and also delete them in the camera roll destination?