Grini, you remind me when I was starting with UE4 a few months ago 
I am afraid this will take hours to answer all the questions you may have, but the good news is you’ll find the answers steps by steps in tutorials and the likes. Look at Youtube videos on landscapes, landscapes materials and layers. search for splatmaps too, even if you don’t use them that will give you some hints. Look into foliage and procedural foliage spawner (this is in the UE4 docs).