Yes, they are made into separate maps for each landscape chunk.
When you export a tiled map from WM or whatever else. You get files named “Output_x0_y0.r16, Output_x0_y1.r16, Output_x0_y2.r16” and so on. Each of these can be anything upto the max size suggested here. Higher res tiles will obviously take a lot longer to export.
Then, when you create a persistent map, and enable World Composition in the world properties. You can import a tiled landscape via the levels window. You can set the scale and such for the entire landscape there and it shows you the dimensions of the entire landscape in km. I personally set the scale to 200x200x1000. In world machine i set the max altitude to 5km, because afaik, thats the largest you can fit in one worldspace. So theres no point making anything higher than that.
Anyway, when its done importing all the tiles, each one is loaded into its own separate level including a level bounds volume.
The persistent level holds the sunlight, sky dome, fog and whatnot, and you can load each world tile individually inside that. Or load up the level files separately to work on them, but you wont have the atmosphere this way unless you drop in some temp sky and stuff. Best to load it in the persistent world and set the level you are working on as the active map.
Hope this helps!