Ask HN: MMO game servers - how are they written?

This is something that has garnered my latent curiosity years. I appreciate that it changes from game to game and can often be dictated by the design of the game, but imagining a vanilla, MMORPG 101, where would I start with an architecture design for this? Does anyone have any resources that discuss commonly-used architecture for these kinds of games? I am familiar with the EVE set up but only as almost a canonical example of how to do it differently...

There is no commercial motivation behind this question, I am just genuinely interested.