That’s the power of having a clear outline of the story and not just writing chapter by chapter as most other LN authors do, not knowing where the story might end up. (Just take Smartphone as an example where the author goes from fantasy to mecha sci-fi to space aliens… *sigh)

Miya Kazuki has an incredibly structured writing process that allows for that type of foreshadowing tens of volumes in advance. Here’s an excerpt from the fan book about the writing process (emphasis by me):

Q: Just to get a general idea of your creative process, how do you write the story, and how far ahead do you plan things? Fantasy worlds like this have an entirely different culture and religious system from what we’re familiar with, not to mention a bunch of colorful characters—how do you plot all this out?

A: I began by building the world the story would take place in, considering the geography, climate, history, architecture, cuisine, fashion, industries, general economy, status structure, religion, their views regarding love and marriage, the relationship with nearby countries, and the fantastical elements. I’m not very good at creating things from scratch, so I based a lot on German culture, while also taking inspiration from places like Austria, Switzerland, the Netherlands, and Sweden. There are also some elements that draw from historical Japan. I think it’s fine to sometimes play it a little loose with your setting as this leaves it room to grow, but you really need the basics in order so that, if someone asks a fundamental question, you have an answer for them. In my case, I read somewhere over fifty books as research before I started. This Q&A session has made me really glad that I did all this, too! Once the world was built, I started formulating the plot. The first things I decided on were the beginning and the end; knowing how the story opens and concludes provides the overarching framework that I can fit everything else into. I mostly write on copy paper, and what I initially did was outline the plot from the beginning of Part 1 up to the very end on an A4 sheet. To give you a general idea, it was something like: “A soldier’s daughter so sickly she’s on the verge of death.” → “Uses her modern knowledge to invent things, which leads to her meeting a merchant with connections to nobles.” → “Finally makes paper! Woohoo!” → “Barges into the temple’s book room during her baptism.” → “It’s discovered that she has mana.” → “Uses negotiation skills and her connections with the merchant to become an apprentice blue shrine maiden in the temple.” → “Saves the orphanage and gets more manpower.” → “The printing industry steadily advances.” → “Draws the attention of nobles during a ritual that requires the use of mana.” → “Finishes a book with mimeograph printing.” → “Some trouble with a noble from another duchy that begins with a Devouring orphan.” → “Separates from family and enters noble society.” → “Becomes the archduke’s adopted daughter.” As I’m sure you can see, I focused the plot on her progress making books and her ever-increasing status. With the broad outline established, next comes the… less broad one. My rundown for Part 1 ended up covering a full A4 sheet, taking up just as much space as the full outline. I wrote all the events that needed to happen between her reincarnation and her joining the temple—things that had to happen no matter what detours she made along the way. Once the more detailed outline was done, I started creating the main characters: the daughter of a soldier, her family, a merchant to serve as her bridge to noble society, someone to connect the soldiers to a merchant, a childhood friend to make paper with her, and so on. I consider the personality and role of a character to be their most important aspects, so names and appearances are settled on last. During the plotting stage, I give characters placeholder names like Merchant A, Childhood Friend (Male), and Attendant 1 (Adult Man). Then, once the characters are in place, I write the most detailed outline yet. What I do here is fill the spaces between the essential events. “This scene needs X, so I’ll write a scene about making X.” “I should have a scene with X before Y happens.” “I want a scene describing X at least once.” These are the kinds of thoughts that cross my mind during the process. Something that often keeps me motivated is wanting a character to say a certain line, or having a particular back-and-forth that I really want to include. The chapters I post to Narou are based on this most detailed outline. In some instances, I might detour a little based on the feedback I get, but it’s usually pretty casual and I’m always ready to get back on track with the important stuff. I don’t really like things being too strict and rigid, since I find that less enjoyable. The idea of being satisfied in the plotting stage and not changing anything is a bit, well… Either way, I try to give myself some leeway when it comes to the story.

TL;DR: Basically she does three increasingly detailed plot outlines before writing the actual chapters and had at least a basic outline of the entire story all the way to the end before she even wrote the first chapter.