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

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.

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