I know this may come across as batshit crazy, but I think I know what Ultra beasts really are.
Look at Guzzlord, for example.
Sun:
"It has gobbled mountains and swallowed whole buildings, according to reports. It's one of the Ultra Beasts.
Moon:"
"A dangerous Ultra Beast, it appears to be eating constantly, but for some reason its droppings have never been found."
There's no way it eats unorganic matter for sustaining itself, especially when it has no fucking stomach and its mouth looks like a portal, so it doesn't "feed" like that.
Think about it: How do the people living in Ultra space find enough material resources and food?
What I'm trying to say here is that the UBs are bioengineered life forms sent to our dimension
with a purpose.
The implications?
The unnamed faller, partner of Nanu and Looker may not have died.
What if she was transported to the Ultra space, taught the people there about our light and how it keeps us alive and then was manipulated with Nihilego to intensify her desire to see the light and help build/summon/awaken Necrozma and raid our dimension?