Let’s make sense of this. Transmigrating into a book, first must mean the universe mentioned has to exist somewhere. The Book is acting as a portal. What about the author(s) who transmigrate into his or her book. Probably, the author was remotely viewing a different universe or channeling into a different realm but is not conscious that what he or she is doing. How did the book he or she wrote without, voodoo or any kind of enhancement, became a portal?? Idk. I’m not a scientist. But I can theorize that it has to do with the state of mind. B-but the book has an ending, thing that have and has yet to happen, how could that be if the universe exist?? Hmmm—the author channel the future. It’s a potential future. What about the transmigrator who dies and goes back in time? I think they actually didn’t go back in time. They die in one universe and enters another parallel universe. How? Idk? Anyone has a theory? A comment? Maybe just some nonsense?
Let's exclude how the universe is or was, because we clearly don't know actual shit about our own universe. Let's talk about how a transmigration which is possible via metastasis in the physical planet or spiritual traversal in the astral plane. This is based on how much light and web novels we've read by now. So when a person returns back in a certain timeline, it varies on the method used. If with body, then parallel dimensions branching theory is proven false, but if with soul, parallel dimension branching is prove true. Parallel dimension branching happens regardless of the observer's (MC) return, because possibilities are endless for the finite observers (not-MC). A good light novel that tackles this is Return By Death. No ones know if it's rewinding timelines or traversing parallel timelines (by here timelines = universal reality). Fuck, I'm not in the mood to explain, but I can sum it up in one word: INFORMATION! Information must have a medium and this medium must be something existent in the universe. Information never truly dies, just becomes unrecognizable it virtually dies, but it's still there in some medium we cannot observe. You can think of transmigration in novels like how the vast the world wide web is. Fuck, my mind is draining. Until next time when my sanity is clear.
I would postulate that nobody ever actually transmigrates, reincarnates, or regresses. Rather I would assume that some cosmic energy wave picks up and carries the frequency of our thoughts and memories and then carries them through time and space to deposit them in another shell. Therefore all such characters are in fact still the body's original self just with an adjusted frequency of thought and memory leading to the belief of a new identity.
Actually, even when the author wrote the ending, it wasn't really an ending since the story do go on and on continuously behind the scenes which is why there are fanfics. What's written in the book will just be a single perspective of the story that was written. So when people actually transmigrate into the story world, they often explore a whole magnitude of things that was not described nor written in the novel before. This is on the assumption the story world exists in the universe.
Falling In Love With The Villainess proposed a theory of its own trope, in that every time a story is penned, a new world is created and a new entity comes into being to enforce its progress. In that story, the 'goddess' had no justification to change the story on its own and has to abide by the story the author wrote as well as change events to fit the plot of the story. At least that's as far as the story goes. Once the story ends, the goddess is free to do whatever she likes.
At the end of "It Seems Like I Got Reincarnated Into The World of a Yandere Otome Game" It's revealed that the Spoiler creator of the game lived in the world that the game is based on and reincarnated into our world and created the game from his past life's memories. The main character didn't reincarnate into the game world, she just happened to reincarnate into the same world as the creator's past life. This story proposes that the world the main character reincarnates/transmigrates to is preexisting. So maybe the author was a previous resident of the world the book is based on and subconsciously draws on their past life's memories to create the book.
If I was transmigrated into a book, it would become an adult book with my dirty thoughts and R-rated humor!
In that case, would the events already happen? Therefore the events and time in which the author wrote about had come to past. If anybody transmigrate or reincarnates into the world the book spoke of, wouldn't it be in the world but far into the future of that world's timeline?
It could be that there's an infinite multiverse, and so there must be at least one world which matches the book the mc is sent to (like how infinite monkeys on typewriters can write the entire works of Shakespeare)