How do strong players know opening ideas, plans, etc? A scenario that already happened many times. "GM X" comes out with this new book on "Defense Y" and this book becomes an instant classic for its lucid, clear explanations of the Defense's main themes, ideas, tactical motifs, etc. etc. Assuming that chess knowledge is learned from books, and that there was no previous such book on "Defense Y", how did "GM X" learn all these ideas, plans, etc? Did he learn by himself, playing and analyzing game after game played with "Defense Y", then got all his conclusions about the opening plans? Is there some secret "oral transmission of knowledge" among strong players, GMs, etc. so that other GMs told him what the plans are? Or else?
This kind of book is usually a work of synthesis. The author looks at the available games and takes notice of the various plans employed. It is a matter of noticing and making notes and then writing up a synopsis. An opening book can be written without prior personal experience with this opening. One can also discover a new plan, either by one's own thinking or by looking at the way an engine plays.

Sometimes ideas are transmitted orally. Bologan's book on the King's Indian Defense is by his own admission written on the basis of the notes he took while being tutored by his guru Lanka.