I’m not sure I understand the question.
Similar to how games have implemented loading behind the scenes, to give the illusion of an “Open World”, one can implement a similar mechanism to switch servers as the user walks on land.
I don’t have much familiarity with actually making an MMO, but this question sounds like an XY Problem. To quote the linked page:
The X-Y Problem, as it is sometimes called, occurs when a person tries to solve a problem on their own, pursues a method that seems like a promising approach, gets stuck, and then asks for help with their chosen method (instead of asking for help with the original problem).
Is it normal for an MMO to use different servers for different cities? The few I’ve played years ago would use one server for each continent (mainly to reduce ping). How would the game benefit by using more servers?