Worldbuilding is the process of building the stage your actors perform on
There are things to act and things to be acted upon.
The characters in your story are things "to act" and the world is the thing to be "acted upon". Roughly, the world is the stage your characters perform on. That's it. Sure, the characters may burn down the set and break all the props but they are still doing that on the stage you designed for them.
The Worldbuilding Process
Authors are in a uniquely powerful position in that they are only constrained by their own ideas and their ability to convey those ideas. They work in pure thought-stuff unconstrained by the laws of physics.
Start with the kind of story you want to tell, sci-fi or fantasy. Here, I will refer to sci-fi stories as ones that could plausibly happen in our universe with our understanding of physics. Fantasy is everything else. Star Wars is fantasy. Lord of the Rings is fantasy.
Next, pick what you want to explore in your world. Perhaps an alternative history where a West Virginian town is magically transfered back to 1632 Germany. Or, what happens when dragons are introduced in WW2? (my answer is they aren't that effectivethat effective.) Most of the time, you will want to reuse a standard set of world mechanics that people are familiar with. This saves on world expository since most people already have a pretty good idea of how Earth works. Many times on WB, we get questions like 'My world is like Earth except...'
Follow through on your changes. If you've made a change that significantly alters how your world operates compared to baseline, you'll need to follow through on that change. If faster than light travel is possible, try to work out how that will impact trade, military and politics. If you can, try to get at the second order effects, ie, the things enabled by the things your change enabled. Note that second order effects are notoriously difficult to predict because of the complexity involved.