Ah, so TL;DR is that each process evals the supplied code and returns the output to the caller?

The universal server may or may not return anything. It just executes whatever is passed to it. In Erlang there are two ways of "calling" (in quotes for a reason). There's conventional function calling which is the regular synchronous style that we all know and love:

  f(10).

This will produce a result and return it to its caller. The other isn't really a call, it's "sending":

  Pid ! 10.

Some process id has been sent the value 10. It may be on this same node, it may be on another node, I don't have to care (sometimes I do though). This is asynchronous. Once a send is done the sending process will continue on (perhaps even terminating). At the other end of the send is a receive (hopefully, otherwise somebody's queue is getting filled up...):

  receive
    N -> ... % do something with this value
  end.

In the case of `universal_server` we don't know what it will become, it's just going to execute whatever 0-ary function is passed. That function may or may not include a "return" (sending a value back to the origin). It could also just terminate the universal server. Or it could temporarily convert the universal server into something else and then become a universal server again.

Slightly different in the low-level details, but conceptually accurate.