SO doesn't run on a single machine, they have a very detailed blog post around their architecture.
https://nickcraver.com/blog/2013/11/22/what-it-takes-to-run-...
2 - still, in 2013 is not 1 server. It's also more than you said now ( add a tag engine server, elastic search, redis, load balancer) and an insane amount of memory on a lot of those servers. Even then, the stack exchange network is more than that.
Saying 1 server, as you did, is not correct and it never was :)