How does fortnite created different biomes on a tiled landscape?

Notice the red line I drew - it is the border between the snow areaa and the forest area.
How do you make that? It’s obviously not square.

Also, you keep using the concept “tile” but it’s not an Epic concept, so I’m sort of guessing what you mean here.
Can you define it so we can be sure we’re talking about the same thing?

Edit:
Successfully attached the image this time.