It’s easy. Shelter is a ‘container’. All apps which are installed inside this container have no access to data outside the container.
If you have a picture insite shelter but no gallery app, you can’t open the picture.
Or other example. Yoy are in your bedroom (shelter) and you can see everything whats in your bedroom. But you can’t see what is in your dining room.
And this goes both ways. All.apps outside shelter do not have access to the files/data insite shelter. That means, the gallery app can’t see / open the picture from insite shelter.
I hope this will clarify. If not, let me know