Why this is a SciFi movie
So IMO Clarence is an alien who is observing Earthlings and assists George Bailey. He has some gadget that transports George Bailey into a parallel universe/reality where the event of him not being born defines that Universe from his pov.
After seeing the ramifications, he decides to come back and Clarence helps him get back to his original universe.
The whole angel and other crap is just ... crap. This is a very good SciFi movie