Lots of arguments about this if you do a search.
I did it for you . . . what I came up with is that there is some consensus that, diving wet, the weight belt should be outside the crotch strap as you have no redundant buoyancy and therefore might have greater need to ditch weight; diving dry, the weight belt may go inside the crotch strap as you have redundant buoyancy. Within that framework are a number of arguments about whether it is, in fact, difficult to ditch a weight belt from inside the strap or whether it is fairly easy, and that will then lead into a discussion of when, if ever, and under what circumstances one wants to ditch a weight belt and what will happen if one does. It goes on and on.
Mackay's book, Dress for Success, prescribes putting the weight belt on the OUTSIDE of the crotch strap in the back and INSIDE in the front. That's a pain to do unless you use Tobin's technique of putting the weight belt on first, then the BC, and bringing the crotch strap around to the side and threading it under the weight belt there, then pulling it down so it hangs in normal position.
I think I just summarized about twelve threads, hopefully fairly accurately.
