It tries hard to keep it simple to change those choices, should you want to, by not shipping too much and relying on the various PSRs to enable easy substitution.
Oh, well, what you're looking at is the default installation of the Primo skeleton, hooked up to a Prismic repository containing the "documentation" for this.
If you're wondering why "Primo"? It's just a contraction on Prismic and Mezzio that also happens to be a dictionary word meaning the lead in a duet. Which one the lead is, I'm not completely sure.