How do you figure out what you want to become an expert in

I pursue what's most interesting for me. Also I consider if there will be a growing demand for it in the future.

Unsure if I have any other than trial and error I found that trying a lot of new things out helped. But, the key was being willing to drop them quickly and move on if they didn't suit me

Let the passage of time guide you. This is something I don't think a lot of people are aware of, but time is a valuable tool. Stay active in the things that interest you and over time, some of those things will drop off as you de-prioritize them. The key thing is to be patient and allow things to organically run their course.