>>58152782
>>58152896
>>58153076
>>58153309
SM doesn't have alternative timelines, it's a different type of multiverse. The (irl) theory is that if the universe is infinite and infinitely expanding then the number of celestial bodies in it is infinite, and if the number of planets is infinite then the number of chances for a planet to be exactly like ours is also infinite. By being infinite, the universe *must* have other planets like ours, planets identical to ours, planets similar to ours but with differences, planets that are like an alternate take on our planet.
That's what you explore in the alola games, that's why when you cross Ultraspace the game explicitly says
>"You arrive at a warp hole that was X light-years away from where you were."
You aren't reaching another timeline, dimension or whatever you call it, where you find other worlds, you are using Ultraspace (the only alt dimension explored in those games) to fast travel to planets that exist in the same timeline as the rest of the game, just extremely far away.

UBs are to space what paradoxes are to time. Necrozma tears space and pokemon that only exist in weird deviations of our planet show up. Terapagos explores time and pokemon that only exist in weird deviations of our time show up.