Sorry if I'm wrong, I'm not sure it's right, but I think that there is a universe where MC played a videogame at 9 yo that transported him in the monster world. Let's call him MC1.
Meanwhile in another similar universe, sunwoo (MC2) was transported in the monster universe only as an adult, switching himself with MC1 and the life that he had built there after arriving at 9 yo (white coat job ecc).
So, till now, we followed MC2 life in the monster universe, while right now in chapter 61 we see instead MC1 that finds himself in the normal world of MC2 (it happens at the same time of the start of the story).
So I think there are like 3 universes, the one where MC1 leaves at 9 yo,
The one where MC2 leaves as an adult,
The monster one where MC2 finds himself and where MC1 had lived in from 9 yo before switching with MC2
Pardon? Now I'm confused again. I thought I understood after someone explained it to me before