How would i make a cinematic universe in books?

I'm guessing you mean something like Harry Potter or lord of the rings which has a large open world with lots of characters.

The best thing is just take inspiration from what you've read; so there could be a baddy through each book, a new challenge through each book, or even 'cycle' through characters and kill one off in the second or third book so you can have new relationships and characters to do different things.

Create a world and then work on the characters in it. What can go wrong? Where can they go?

Use the same characters, make refrences, and keep notes. It's not that hard.

You don't.

The word cinematic means "in the movies."

The term "cinematic universe" is just a shtheel term for sequels ad spinoffs, an invention by pretentious people for something that had been done for decades in pulp fiction, by which I mean the books, not the movie.

Heinlein called his a "future history" and you can find dozens of other scifi authors who did similar things. Most of the time, they simply reused the tech which they'd invented for some other story, rather than go to the hard work of coming up with a new way of star travel, some cool new aliens, or having robots.

Asimov stopped even bothering to explain his three rules of robotics in some stories, because most of the people buying the magazines were buying them for his robot stories.

This is as simple as referencing something also mentioned in another book, a spy organization, a fictitious historical event, an invention, etc...