I dove into world building the day I had a dream about this massive, sky blue bird flying. That was the whole dream. Soon he was the Aslan figure in my world and I had a story setting about a Depression-esque country from which the hero would journey blah blah blah.... Soon it developed and a year later it's a solar system with a history that I keep revising.
Inspiration also comes from "What do I wish this reality was like?" and then you change it. I wanted my characters to be able to breathe in space. I wished we had background music. Insects are annoying. Etc. Now there's enough air in space to breathe, the air emits background music pseudo-magically, and there aren't any bugs or rodents on the main planet. I'm very free wheeling when it comes to creation. A lot of people want things that make sense down to nitty gritty details. (I learn a lot from them.)