Each landscape in a world composition level is a tile. It behaves like a tile and hooks up like a tile - provided the size is correct.
together they form the whole (world).
The regions in any game, not just fortnite, don’t usually have a “solid” border.
They transition gradually.
You can achieve this as explained above with a variable layer material.
you manually paint the specific landscape to have the snow layer with whatever transition you like and finally merge it to the same material used within the other tiles/maps so that it is in fact seamless.