If you're looking for an exemplary server architecture for MMOs, look no further than UO. That game came out in 1997 and supported worlds with thousands of players and millions of persistent objects. Players could build their own houses within the game world, decorate (and later design) them to an incredible degree [1], and have other players visit (welcome or not). Players could run their own shops within their homes and sell their own hand-crafted, signed goods (weapons, armour, clothing, furniture) in addition to any found treasure they wanted to offer.
What made UO so impressive, technically, was how they accomplished all of this on such modest hardware as was available in 1995 [2]. This second link is to the GDC postmortem of the game. If you're interested, I recommend you check it out. It may not answer all your technical questions but it's extremely interesting nonetheless. The story of how they had to shard the game is part of the talk.
I'm pretty sure they did not use any off-the-shelf databases as the performance would have been terrible at the time. Everything would have been custom. The custom networking protocol is extremely reserved (not chatty), since it was designed for dial-up modems. Even when you run the game on modern hardware with broadband, the game uses on the order of a few megabytes per day of traffic.
[1] https://duckduckgo.com/?q=uo+house+decoration&t=osx&iax=imag...