I’ve seen the musical play and the movie, but I’ve never read the books. Is it ever revealed what sort of being Mary Poppins is?
She obviously has powers, but is far from omnipotent, so I’d assume she’s some kind of witch. OTOH, she blends in perfectly with humans and seems to relate to them. Is it discussed where she came from (a Hogsmeade type place, perhaps)? Or if Bert is what she is?
I don’t believe P.L. Travers saw fit to explicate her origins very much. I believe at one point one of the magical creatures they meet addresses her as a daughter of the Sun although whether that was meant to be literal or not is not clear.
As it is, you can make up pretty much any origin you like for her as there’s very little official canon to contradict you.