To the best of my knowledge, the most common parallel universe theory that has any actual real traction in physics is the "many worlds interpretation"
Which is basically that any time some sort of quantum event is observed, the universe splits into multiple parallel universes where every possible outcome of that event is realized in its own universe.
Now people take that and run with it and make up all sorts of pseudoscience bullshit where those splits happen anytime someone makes a choice, or some pseudorandom event like a coin flip or die roll occurs. That's not really what it's about.
This is about wonky quantum physics, radioactive decay, collapsing wave function type stuff. I'll be honest this is high level physics shit, I only kind of understand some of it, which is more than probably 90+% of people out there can say, for most people it probably means about as much as if you came up to them and started talking to them in a foreign language.
So that means that all of those parallel universes are going to be following the same laws of physics since they all diverged from the same universe.
That means that flying reindeer and traveling around the world in a night delivering presents down chimney and such is probably a no-go.
As far as there being a universe where some weirdo named Santa Claus decided to live at the North Pole and build toys, maybe, but probably pretty unlikely. I have a pretty hard time imagining a version of the world where different quantum outcomes would lead to that. Would, for example, a single uranium atom decaying or not decaying make that happen? Probably not. Of course, untold millions of tiny events like that can eventually add up to some big difference, but I still have a hard time imagining a situation where that would be the outcome.
Like I think someone in this thread already said, there are an infinite amount of numbers between 0 and 1 (0.1, 0.2, 0.001234, etc,) but none of them are "2" some things are just impossible.