The scene is very procedural in many areas, the placement of the trees, landscape material places textures based of the angle of the slope, which is pretty cool. There are some pretty cool examples of how to do it on Epic's free learning stuff, I learned how to do this off the "vehicle game" content example. Everything from colour to slope angle is controllable through a material instance, so I can quickly experiment with changes without a hassle. The colour of the grass mesh is also controlled through the angle the mesh is on,which is awesome, and while not really accurate to nature, I can create some interesting colour variations across a whole landscape. Like here (but not orange):
