I was confused about 무엇이 vs 뭐. Turns out 무어시 is preferred for written and formal use whereas 뭐 is better for spoken use.
So, why is it not 속에? I always thought of 안에 as in a room or building and 속에 as inside a box or dresser or whatnot. Is there a more subtle difference?
The difference between 안 and 속 is that 안 is more physical and 속 is more abstract, used for things not so palpable.