He’s on a different planet in the far far far future in the same universe as his Earth.
He discovers that Earth continued advancing to being able to do things like space travel, sci-fi experiments like creating new beings and such. However, at some point the humans of Earth lost hope for some reason and returned to Earth and became shut ins. No I’m not kidding.
The planet he’s on is the result of one of the experiments. The planet was abandoned and the tech on the world became disused and basically some of it broke, and long years after basically resulted in the current world.
Dungeons and Dungeon cores? Laboratories to creating new creatures, however they are now malfunctioning and sending out monsters. The current continent he’s on, is just the southern half of the planet there’s a northern half. The Northern half has even bigger versions of monsters going around.
All the races we see in this series, all created and flourished. And can procreate with each other. Souma is an old human, the humans on this world are new and don’t share the same ancestry as Souma as they were created for the most part initially and procreated and resulted in the world it is in story.
Magic?
NANOMACHINES, SON! No, I am not kidding. Magic is handled by nanomachines. The cursed ore is basically a clump of nanomachines.
Who is the Demon Lord?
A vocaloid. No I’m not kidding. A vocaloid program that Souma knows about from his life on Earth somehow got evolved into its current form and gets put into that position leading refugees from the Northern continent. And they are the demons but are actually fellow experiment races like the rest. But there’s of course a language barrier.
How many wives does Souma have?
8, he is shared between the 8 wives equally across the 8 days of the week. And he has multiple children. 7 of his wives become queens, while the 8th stays a commoner. He has multiple children with them, mainly so his bloodline spreads in order to handle the future tech buuuuut
Does Souma die eventually?
No. He somehow found some way to extend his lifespan in the last volume but it’s never explained how exactly. Just a hand waved explanation, probably alluding to some random super science device that does it. No I’m not kidding. So partially him needing to spread his bloodline is kinda moot point if he’s still around.
Souma and Earth:
In the last volume, Souma finds some way to travel back to Earth, to possibly fix problems there. Never explained how or really why he wants to fix Earth in the end. No I’m not kidding, after he learned how Earth became shut ins and declined. He for some reason thought he needed to fix Earth somehow. Never explained how he’s able to travel there besides possibly finding a spacecraft that got left behind.
Does Souma finally retire from being King?
Yes. His son takes over while Souma becomes a researcher. Why? He was trying to look into any tech that got left behind. Which hand waved answers some of the above. And somehow leads to him traveling to Earth and to fix Earth….no. I’m not kidding. Though it was in a short blurb that described the above.
Also his son proposes and marries Carla. Remember Carla? Liscia’s best friend who became their slave and serves as a maid? Yeah…Souma’s son falls in love with Carla. And when he took over as king he kept proposing to her. Carla finally accepted. But here’s the thing, Carla helped raised all of Souma’s kids.