Let's start the level construction process. There are many ways to begin here, but it's a good idea from the outset to develop with a clean workflow in mind. By clean, I mean an organized, structured, and easy-to-maintain workflow, one that remains maintainable for projects of many scales. First, delete any and all objects in the new scene, and then create a single empty GameObject named root by navigating to GameObject | Create Empty from the application menu. This object will be the topmost object in the hierarchy, from which everything else will be a child directly or indirectly:
Next, create a new child object named env, which will contain all environment pieces. Ensure that both the root and env objects are positioned at the world origin at (0,0,0). I typically position the world floor...