In my experience, the best key for unlocking the mysteries of your make your believe world is the characters. They hold all the answers as to how the world exists and why, because they already live in it — yeah? All you have to do is start asking them for the answers.
Okay, that sounds a little existential, but I really mean it. If you’re uncertain about the shape or your world, explore your characters. What’s character A’s backstory? What does he/she like to wear? Why? How did he/she get those clothes or that type of spaceship or that particular magical skill set? The more you answer these questions, the deeper and richer your world will become and the more it will make sense to your readers.
Your characters will also provide you the level of world-building you actually need to include in your story. A lot of writers I know tend to skimp on the world-building in early drafts (myself include). Consequently, my feedback on these drafts tends to be full of questions on how stuff works and why. A lot of these questions occur to me because I don’t understand why a character is behaving a certain way or how they had access to these particular resources, etc. Which means, that the answer to these questions will come through the characters themselves.
There are also those writers who love to world-build so much that they include too many, and usually pointless, details. Again, the solution here is to stick as close to the characters as possible Would character B, a teenager sneak thief and orphan, really know the name of the son of the cousin of the regional governor of Outlandia? Yeah, probably not.
The best visual metaphor I can use for this idea comes from the super-awesome movie you might have heard of known as The Avengers. There are many reasons why this movie is so good (the biggest of which is because it was written/directed by the Geek God of Awesome known as Joss Whedon), but one of the most critical reasons is because of all the other movies which came before it. Think of those other movies, Iron Man, Thor, Captain America, as really extensive backstories, all of which were necessary for The Avengers to make sense and for the characters and world to be believable. Do the same with your characters. Answer all the questions, explore of their reasons for being. And then remember to only include the details that really matter in your manuscript for your Avengers story.