I mean the process used of course. The modern day mill is used quite frequently at the factory I work at, and as an engineer, its interesting to kind of see the evolution of various things as a result of better tools. Examples of this include milling becoming automated via CNC machines. However, if people from years past wanted to do the processes that we typically use a mill for today, how did they do it?
Do you mean milling as in machining parts? Or as in cutting timber?
If you mean timber than a saw pit is what you're looking for.
One guy above, one guy in a hole down below, and a saw handled as a team. (though this example seems to be freestanding)