When it comes to worldbuilding, I live by the Iceberg Rule: Most of it goes unseen. Another way to look at it is the corollary to “Show, Don’t Tell”: Know, but don’t show.
What this means, in practical terms, is I sometimes ask myself questions that knowing the answers probably won’t impact the writing itself. At least not directly. But asking the questions, and then forcing myself to have the answers, enriches the worldbuilding process, and hopefully, enriches the writing itself.
How did prehistoric people spread out onto all the continents? In our world, by around 11,000 BC human beings had more or less filled out the habitable parts of the world, migrating out from Africa into Europe and Asia, across the Bering Strait into the Americas, and island hopping through Indonesia, Australia and the Pacific Islands. All this before any significant agriculture or animal domestication. In the worlds we build, have you figured out the logical A to B to C of diaspora? If you have multiple races– or, more correctly, different intelligent species– have you worked out where they each started and how they spread?
Who were the big thinkers of history? Even if you have a relatively boilerplate Medieval/Renaissance style fantasy world, the steps necessary to get there still matter. It’s important to know who the Caesars and Alexanders of your world are to shape the politics, but what about the building blocks of culture? Who was the Euclid or the Pythagoras? Who was the Socrates, the Aristotle? Who was the Homer or the Sophocles? And that’s just the classical: advance things further, and you have to ask, who did the sort of work done by Galileo, Newton, Kepler, da Vinci, Robespierre, Descartes, Shakespeare?
In a world with magic, what does the periodic table look like? This is a specific example, but it hits the heart of how magic and science interact, in a world where both exist. If magic is real, what does that mean in terms of science? Can it be explained scientifically? Does it change what we know to be the concrete building blocks of matter? Take for example, in Thorn of Dentonhill, a key plot point involves a magic-altering metal called napranium. Now, I could take a lot of different paths of what this metal is, on an atomic level, even if this never becomes part of the text itself. For example, napranium could just be, say, gallium, and gallium simply has magic-affecting properties that in the real aren’t an issue. Or, napranium might be an unstable isotope of gallium, and there is interconnection between magic and radioactivity. OR, on a subatomic level, protons and neutrons might ALSO have a magic-based aspect, so while regular gallium has 31 protons, napranium has 31 mago-protons, and is a fundamentally different element. Meaning the periodic table itself needs to be expressed in a way that doesn’t apply in our world.
These are not the kind of questions that really get addressed in the average sword & sorcery type book, nor do they need to be. But they are the sort of thing that I think about, and thus shape the underlying texture of my worldbuilding.