Remember that all is opinion.
As worldbuilders we often confuse what’s “real” or universally true in our conworld with what its inhabitants believe.
Buddhist doctrine distinguishes between two types of truth; the relative and the absolute. All truths derived from perception are relative, as they originate from a perspective. At the present there are thousands of worldviews, each simultaneously providing conflicting truths. Religion gives us monotheism, henotheism, polytheism, and pantheism; philosophy gifts us a hundred schools of thought, and science (which until recently was undistinguished from philosophy) a thousand competing theories. We can safely assume that over the coming centuries the knowledge of today will seem as archaic as mythology. As individuals within a universe, we have a limited perspective and can never know everything or anything in totality. To an individual within a universe, all truth is relative.
In worldbuilding the opposite is true. We’re creating a world in which we’re not on the inside; we’re the God in every sense of the word, aperspectival, omniscient, and omnipotent. The world is us and exists inside us, and there’s not a single atom, city or star that exists without our knowledge. Everything we believe about our world is an absolute truth. Like Gods, we create the world in our image. A little too much in our image, however, because the occupants of our worlds seem to have near-omniscient knowledge about it as well.
Take language as a simple example. In a modern, literate, interconnected world, we have standardized scientific names for plants. In the ancient world, a plant that grows across an entire continent will have hundreds of names, even within continuous civilizations. The same applies to geography, fauna, cities and empires; there are no universal names. Relativity directly affects the world – competing cultures will have drastically different versions of what happened a hundred years ago, which in turn influences the present. Relativity is especially pertinent when it comes to the metaphysical. Most conworlds have a cosmogeny – often the cosmogeny, which isn’t so much a myth as it is an account of actual events – and all the inhabitants of the world believe in the same version. This is very different from the real world, where there are thousands of coexisting mythologies. Myths are metaphors for the truth, and multiple metaphors can exist simultaneously without conflicting. Yet in the majority of published conworlds, the spiritual appears to have been standardized.
Some worldbuilders will do this consciously or for the sake of simplicity, and that’s fine – it’s your world, just give me a good reason why it works. But most conworlds are populated by perspectival beings, which leads to inevitable differences. I see this as a great creative opportunity. You don’t need to restrict yourself to a single worldview; your world can be host to a wide variety of mythologies reflective of its differing cultures. Gods, spirits, and energies can be mutable and mutually compatible; contrasting cosmologies can draw from a single truth. History is fluid; magic is diverse. Ideas abound.
My process is to create a truth and let the occupants of my world try to figure it out. This is more reflective of the real world, but organization is key. My solution is to divide my writing into two categories – canon and artifacts. Canon is absolute – it describes the world as it actually is, providing a consistent foundation for reference. Artifacts are writings (tablets, scrolls & codices) produced by characters inside the world. As both types of writings provide insight into the world, I link them together. Canon articles about historic events include links to their various in-world histories, while in-world cosmologies can refer back to their canonical metaphysics. This works for me. Find something that works for you and stay creative.