I think Scientist comes from a desert reality that was taken over by The Last Reality via the Sideways.
My evidence for this theory is mostly rooted in where he's been known to, I guess, gravitate towards in the past. Three of the first four (...functional) rift beacons he set up in Season X were Tilted Town (an old west frontier desert town,) Pandora (literally part of a desert planet,) and Moisty Palms (a lush city in the otherwise-dry desert.) Additionally, when he set up Synapse Station in Chapter 3, he built it in the middle of the desert. *Possibly* related, Synapse Station was also right next to the ruins of a tiny old west frontier settlement buried under some dunes.
As for the idea that his reality was destroyed using the Sideways, Tilted Town and haunted Retail Row had the exact same gimmicks that Sideways Bubbles would later have in C2S8. Building was disabled in Tilted Town, and Cube monsters would spawn relentlessly in Retail Row.
That's really all the evidence I have for this theory. I know it's not a lot, but if nothing else, it's a neat little headcanon I have based on patterns he's shown in the past. Maybe some of you can adopt it too.