So, I apologize if I just don't answer your question the way you want it answered. Here's some rock-solid-24-carat-gold DMing advice:TLDR: How does Commerce work in your games and how do you prep it? Do evil empire have commerce at all?
If the players don't care, then you don't need to care/know.
If you just like world building, then do whatever, but from a practical standpoint virtually no PC is ever going to ask you for these kinds of details and if they do, it'll be easy to just come up with an answer on the fly. Especially if you're going with a vaguely pastiche medieval-fantasy environment.
But if you really like this sort of stuff, I suggest you take a quick look at: https://gamingballistic.com/wp-content/uploads/2018/11/Medieval-Demographics-Made-Easy-1.pdf