According to spoilers from below like way below , the story will mainly focus in the OG modern world but idk they could be misunderstanding or wrong
Right, it's hard to tell because the raws itself are in Korean. But as of now (as far as i read the korean ver), the MC (the guy who enters the fantasy world) seems to be going back in time (fantasy world) in a male form, where William was still a kid. Although I could be mistaken because I'm not sure if that kid is actually William. Meanwhile, the modern world continues to make cameo appearances as usual.
As for the ending itself, I still can't picture what will happen—whether they will come back to their respective worlds or never come back and stay. But for now, I think it's more likely that the first option will happen. But I don't know for sure, it just more make sense for the 1st option to happen.
How many chapters are there for the raws?
I'm not sure about other sites, but the usual site that I always visit so far has 55 chapters. It should have 56 chapters, but it hasn't been updated since May 2nd.
I can't quite picturing what will happen at the end, this is new, because usually when the MC from modern world enter the fantasy world, the other soul was vanished/gone. But this one, both are present and literally switch soul. Plus, they have their own story for their journey.