Cant find it anywhere ;o;

I don't think there has been any official explanation. We know they're supposed to be from Earth because of the Earth language textbooks in the Book Nook and the Subetapedia entry for Veta says -

I sometimes wonder if any members of various Subetan races migrated to Earth - maybe the pointy eared Tehb were thought of as elves.

I believe Keith once said in either the forums or an Ask Keith livestream that humans just came over in a rocket ship. Googling a source is failing me on all fronts however, but this is the information retained in my mind.

Oh I remember what keith said actually--a meteor hit earth and catapulted a bunch of humans to Subeta, where they crash-landed and formed settlements

jk but what posted feels familiar to me. I think humans did indeed migrate from earth to Subeta. no idea about the other NPCs who are clearly human-adjacent but not fully human, however.

It would be good if we knew which NPCs were humans (whether they arrived here or are descended from the ones that did, I don't know how long ago that was) and which ones are native species. I assume all the NPCs with pointy ears and/or other non-human features are native Subetans (of which there are different species or races like the Ji-Meneb and that race from Shengui Guo with the multiple eyes) and the ones with round ears are humans, but it's hard to tell with the ones whose ears we can't see. And also, I don't remember if Shinwa or the Oracle have pointy ears or not, but Maleria does and she's apparently from the same planet as them (I think it was called Selinis or something?). And I also don't know about the Wizard.

That meteor story sounds silly but I guess maybe there was a rift near Earth and it sent them through that, since the rift was there sometimes before it opened more permanently, and I remember reading something about there being other places with smaller rifts that sometimes open as well (I think it was mentioned during the 2013 plot, I don't remember a lot from that but I read through some dialogue from it once).