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 (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).
SPOILER
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For now it seems that MC2 died in the monster universe :'(

No need dr'gs, i feel high asf with the explanation going on. I'm confuse ╥﹏╥
Can we do basic explanation for my slow ass pleasee
So, 2 MC
(1)Real World
(2) Monster world
+ Both exist in real-time. Both are real. Separate entity that just got exchange world. So, who's who in chap 61??