"the App should call the OS interface where only one file is reachable"
But what "one" file? It has to read the file/library in order to see those files. It also has to read it in order to save any photos taken.
Same with your desktop. When you upload a file in your browser, that window to select a file is from your MacOS Finder (Or Windows file explorer). You select one or many files. After you confirm, only those selected files are reachable.
When you save a new file, it goes to your filesystem without having access to other files there. This is basic OS filesystem management we have in Desktops for ages, and mobile OS struggles to adopt.