Let’s see how to get this result, starting from Instant Terra.
Use some generators and tune your terrain until you get the expected result in Instant Terra.
Generate masks, they will be used to set your textures in Unreal Engine.
Then export all: your terrain (as a height map), your masks and the color map.
The first mask is used in Unreal Engine to apply the grass texture.
The second mask is used to apply the rock texture.