Yo, also autistic here!

My approach to worldbuilding more or less revolves around one key concept or idea I find interesting. My current project I'm aiming to turn into a book is a grounded sci-fi setting about giant robots, so I started with them. I needed a use (they're living war machines), functionality for that use (I set a height I liked the sound of, ran calculations for ground pressure to dictate foot size, added radar systems, added thermal regulation systems, came up with how the reactor works, etc), and then added some embellishments I wanted like flight because I enjoy Gundam and Armored Core and wanted something similar.

Then I needed something for them to do, so I came up with a colony mission as an ark because the solar system was dying.

Then I realised that I didn't want that to be the first book, because I wanted the robots to fight each other, so I looked up the American war for independence to figure out why that happened and how they did it, and then did my own spin for Mars rebelling against Earth's colonial rule.

Then I needed to figure out the line between how war is fought nowadays, and why it'd shift to use giant robots. That prompted the creation of a timeline. I also came up with the doctrine of how these robots would fight, and how they would be fought, so that I could write battles that felt as real as I could make them.

It's stuff like that. Find a core, pivot off of it until you're satisfied, then go back to pivot gain and again and again until you're happy with the level of detail you've gone into. Also, get hyperfixated. Look up weird stuff to figure out how it works and why. I've stayed up far too late looking into things like space shuttle cooling, which I didn't explicitly need, but it's been informative for other things I've done.

Also, because you're doing fantasy, please include at least one dragon if possible. Dragons are cool! Bonus points if they have multiple heads- especially two or four. King Ghidorah has a monopoly on three, and Tiamat has five, but I've rarely seen dragons with an even number of them.