The latency critical stuff traditionally happens in dungeons or other instances, precisely to get those players on a shared physical server. You just have a fleet of servers that each can handle X instances, and have a queue in between. And conveniently player state can just be synced in the loading time before and after the dungeon.

The bigger world is handled by slicing it up, but you still have a lot of communication going on with central databases for stuff like inventory management, chat, quests, etc. so you would probably try to keep all that within your own server racks.