Community manager from Perforce here. Basically it's what Sytse said; we do bi-directional replication between a p4 server and a git repo. You can carve off any hunk of a p4 server and surface it as a git repo. What we've seen some people do is push a monorepo into a p4 server and then carve off smaller repos that are more tightly scoped. Everything still goes back into p4 though, so you can still get the benefits of a mono repo.
The neat thing is that multiple git repos can consume the same pieces of a p4 server making code sharing between git repos seamless.
Exactly. For game companies in particular, the code and the digital assets are dependent on each other. It's a bad day when you run your code against models that are missing bones the code expects.
We actually have an open source tool that allows you carve off parts of your Perforce server as Git repos. The repos can overlap in Perforce allowing you to share code seamlessly between different Git repos. You can generate new repos from existing code easily and can even generate shallow git repos that are usage for development.
The neat thing is that multiple git repos can consume the same pieces of a p4 server making code sharing between git repos seamless.