Erlang VM is written in C. It has a concept of a lightweight "process" that can share memory with other processes. It spins up a real OS process for each core on your machine, then a scheduler let's you run thousands of async Erlang "processes" while managing who gets access to the CPU. In web frameworks like Phoenix, each HTTP connection is a process. So you can serve thousands of users at once with tne default configuration. You could go ahead a write a system like that from scratch in C, but you'd be wasting a lot of time.