Building foundations?

Okay, this may be a dumb question and maybe I’m over thinking it, but when you build a modular building, let’s say a shop of some kind, and you want to put it on the mils plate, do you build a plate foundation (like build it on a 16x16 plate) and secure that plate to the top of the mils plate, or do you use the top of the mils plate as the floor/foundation for the building? I’m thinking on top, yeah? So that the building sits above any path or road way? I think I just answered my own...