Uhm, actually: [Open-source] Realm Crafting - MMORPG Editor
Well, you could always download a ready scene from sketchfab 
But seriously, I do not understand what you want to achieve. What are the requirements of the school project exactly? Also what school: high, grad…? And on your side: do you want to learn or do you want to make something? You need to narrow stuff down.
Or is it that you already decided to adapt Porter Robinson’s website and you do not know where to start? I would do as you intended to do: make the landscape in Unreal [or Blender], find a way export it to babylon js somehow and make it work What is wrong with that?