PostgreSQL uses a mailing list and Google uses Gerrit to name two "major" counters. There are plenty of big projects and orgs operating under different models.

I'm talking about stuff like Gitlab, Bitbucket, etc. What I am saying is that most of the paradigms that GitHub has have familiar if slightly different implementations in other places.

But they aren't the only ones, nor the only ways. It's not necessary to know other paradigms, but knowing of their existence is nice, but also they should never be discounted as lesser methods of doing source hosting and code review--especially when used by large orgs and projects.