Most people know this I'm sure, Its the same technique that comes with starter content & often mentioned in material studies. There are actually 2 ways to make this material but this is the common & better one.
This is the Macro Variation material you just multiply it into your landscape material towards the output or in your landscape layer as required. It helps hide repetition.
Macro Texture - You can find it in Unreal Starter Content.
End Result.
You can of course use any texture that you find works. In this case its CellNoise(Voronoi)while looks out of place on grass works well in deserts.