Can't speak to how GitHub does it, but in every Rails org I've been at, eventually we create secondary services that own a specific domain, and the primary app

Can't speak to how GitHub does it, but in every Rails org I've been at, eventually we create secondary services that own a specific domain, and the primary app becomes a gateway that clients use to talk to those services. It's rare to spin up a new service in a new language/framework. A typical pattern is to migrate the service within the monolith into a Rails engine, and then move that to a new app where the app mounts only that engine, or one can find a way to make the monolith deployable with only certain engines mounted, like with an env var. A Rails engine is basically a self contained Rails app, including routes, which you can mount inside of a host Rails app at any route if your choosing. They're usually used to build reusable libraries, but this use case also works very well.