How to make the player move around the map on the server side?

Question

Game development

I am learning to make a client-server game, I was able to move around the map in one plane (without elevation changes). And the question arose: how in games do objects move around the map in two planes, that is, with different heights?
I had an idea to make a "height map", roughly speaking a two-dimensional array of heights for polygon points, but in this way it would not be possible to implement two-level maps, for example, a house with two floors.
I will be especially grateful if you tell me literature on this topic, until I managed to find it myself