How the real social networks web apps use routing?

They use client-side routing? but they also have routing in the server right?

For example when I go to someone's profile and the url looks like "https://facebook/someone", that's client-side routing? because I don't think they have a path to each user in the server, but also you can refresh the page and with the client-side routing its a problem.

I've been reading about server side rendering and isomorphic web apps, but still confused about the routing. Can someone explain me?