So we know the people of Ultra Ruin are just parallel universe humans. The rest of them fled to space to flee the Guzzlord that appeared on their world and took advantage of their polluted, cluttered society.
Ultra Forest has the Kartenvoys, who are old men that use Kartana and train in kendo. Therefore it must also be an alternate universe version of the Pokémon world. But does that mean Kartana are native to that version of the planet? Could there be Kartana somewhere in the usual Pokémon world that are native?
We never see much outside of Ultra Megalopolis, but since promotional material has referred to the Ultra Recon Squad's people as "humanity" I think it's safe to assume they are ALSO on another alternate Pokémon world, one where the entire planet's history is involved with Necrozma. Again, that brings up the question, could there be native Poipole and Naganadel on our planet?