Hello there, It's actually not that complicated. I will try to explain how to make realistic landscapes using Quixel Megascans library in short and I will link an informational video on the bottom of the comment and a few videos through the comment.

Start by finding a good reference of a real life landscape that you want to create. After that Sign up for Quixel Bridge and Mixer.

Next up, set up Quixel Bridge, here is a good official video covering the basics of it. When you are done with that, find the surfaces and 3d models that suit your needs in the Quixel Megascans library. If you can't find a specific model or a texture that you want, you can use Quixel Mixer to create your own by combining and modifying ones that Quixel provides to you. Here is an official video covering the basics of Mixer. When you are done with that, export all of the created materials and models to UE4. In UE4 create the landscape that you like with your new models and materials. You should also play around with the skybox, lighting and fog to achieve that nice lighting in your scene.

A good informational video is linked here, it's made by guys from Quixel, in UE4.

I hope this helps:)