How to choose which product (speciality) or asset class to go into?

As a prospective investor(AM/HF) ,there are so many products and asset classes to choose from-Equities ,Fixed income,Equities derivates ,credit ,Rates ,Swaps etc etc. How to know which one is the best for me. And do people get assigned a product on their first job and then develop their expertise. If you get an equities job at MS ,a Interest rate at JPM,Fixed income at GS how to choose which one to take because your future depends on it as you develop your speciality. Oaktree is great in Credit investing ,Pimco best in Fixed Income and they will recruit respective people from top BB. A credit trader cannot become a systematic equities trader after a few years. Hope you get What I want to ask.