Why would they not use the same repo? It can easily be a standard product - look at the example further downthread of the unreal engine: All clients get access

Why would they not use the same repo? It can easily be a standard product - look at the example further downthread of the unreal engine: All clients get access to the code. 2 private repos and 90657 users. That's an extreme example, but we also have a single repo that a lot of collaborators get access to.