> some commands take remote branches as `origin mybranch`

That isn't a single argument, that's two arguments. If you look at docs for some commands which accept `origin mybranch`, git fetch for example, it says the first argument is `<repository>` which can either be a URL or a remote name. In your example it's a remote called `origin`. The 2nd argument is then a `<refspec>` which is a bit complex - it specifies what to fetch and where to fetch it. So in your example `mybranch` is shorthand for `mybranch:mybranch` (i.e. fetch `mybranch` from `origin` and update local `mybranch`). You can even do `git fetch origin mybranch:mybranch-local` which would fetch `mybranch` from `origin` and update `mybranch-local` with it.