Hello Beardies,
You’re familiar with the iceberg analogy: only a small fraction of the whole is visible on the surface. It applies to people, art, sports, nature, and it applies to world building. You, dear writer, should have, floating about in your brain, a berg-size mass of knowledge about the world in which your story takes place. This knowledge informs your writing and adds layers of richness to the story. Even so, only a small portion of that iceberg will surface in the story. And that means you have to make choices.