I mean, each author handles subplots differently. A famous example is from a book I reference a lot - House of Leaves.
The book is pretty much a plot within a plot within a plot. The story starts off about a man named Johnny Truant, who excavates the home of one of his neighbors after said neighbor passes away (an elderly man named Zampano). Johnny reads through notes, documents and other written works recorded by Zampano himself - they are personal thoughts, criticisms, and observations regarding something called The Navidson Record. The Navidson Record is a series of recorded documents, text, audio, and video, all documenting the strange happenings within the home formerly belonging to the Navidson family, which Will Navidson (the husband and father) quickly realizes is quite literally bigger on the inside than it is on the outside - with the inclusion of a basement containing an endless stairway that stretches for miles and miles, which he eventually explores with his friends (at the risk of their own lives). So The Navidson Records are discussed by Zampano in his notes, which are eventually read by Johnny Truant, who ends up devolving into manic insanity as a result of the paranoia and fear he garners from reading the notes (plus his mother happened to be kept in an insane asylum, as pointed out in her letters which she sent him regularly).
The book is just as much a confusing maze as I've made it out to be - it explores the plot through every method available and possible, including differentiating fonts based on the narrator, excerpts from other books, interviews, and writings (some that exist, others that don't), scrapped photographs, letters, codes, footnotes, newspaper clippings, the list goes on. That's literally the best way I can explain the plot without 1.) spoiling too much and 2.) coming across as a crazy person. It is a very fascinating read and is one of those books you can read over and over again and still discover something new each time.
In the example of my own series, there are characters that start off in book 1 that disappear through books 2 and 3, but when they do make appearances in these books it's to set up for later books where they either redeem themselves or become bad guys. Certain characters "overlook" the plot, making sure other characters "progress" through it correctly; other characters break the plot entirely, but still make sure to keep questions relevant and interesting. Ex. there's one character who comes back as a villain in book 4, but in book 6 isn't a villain and is instead a broken human being, and by the 10th book they're looking for ways to humanize themselves - and this all happens while the main plot is still progressing, and their arc eventually ties into the main plot by the end of the series, acting as a catalyst for many of the other characters' decisions.
IMO, so long as everything comes together in the end, you can play with the subplots, their narratives, and their contexts as much as you want. Just keep in mind that everything should tie together, no questions should be left unanswered (unless they're interpretive questions like the ending of Inception) and there shouldn't be any major plotholes that ruin the entire plot as a whole.
