i too wondered this for a long time.
for now you're not supposed to understand. it comes up way later in the novel. the details of kim rok soo's backstory are also explained way later. even in the novel, it just sorta drops you in w/o much background and it's all slowly revealed as it becomes relevant to the progression of the story. for now just go with it.
Expanding on this it's actually pretty amazing how well thought out everything is. Things that happen in the first couple chapters that seem mundane suddenly end up being relevant 300 chapters later. The author's really good at throwing in these great plot twists that don't feel like they were just dropped in because things were getting boring and predictable since the twists get foreshadowed so early.
So if you ever come across a piece of the story that feels a little weird keep it in mind because it might suddenly be relevant much, much later.
Such as the tree that was... SUPPOSED to turn white ༎‿༎ But yeah, this author is crazy good at introducing the smallest of details and then making them relevant in a way you wouldn't have even thought of until it the moment it actually gets revealed, and everything clicks togethet. Author-nim blows my mind constantly haha
So the MC was transported into a book about a guy who transmigrated from the real world. Does that mean that Choi is actually from the real real world or the book’s “real world” which is really not the real world that the Mc is from?