I created this scene mainly using the Mars pack from Brushify, I got the pack for the alpha brushes, which are very powerful for sculpting landscapes.
Get it here: https://www.unrealengine.com/marketplace/en-US/product/brushify-mars-pack

If you would like to see how this scene was created, you can check it out here: https://youtu.be/xbUD518jV74

Sound on