... there is something called an Editor.
Starting with an empty space, we add cubes (or whatever shape your heart is asking for) to build up the essential outline of your virtual world.
When putting one box onto another, some faces become hidden and things might get quite complex. Instead of masking those faces out by hand, we simply press one button and the Editor does everything to optimize our world for rendering.
While our new reality emerges, we want to colorize it. So we do by simply adding textures and colors to the walls we have created. The Editor provides a neat tool for that, called “flood fill”. Finally adding lights to our world will make it more realistic. Remember that all lights in my engine are dynamic - so they can be altered and moved within the game at any point.
And if that does not hit the eye, it will be up to you to build your own worlds in a few simple steps and maybe create a whole new game out of it, or at least have some fun setting your creativity free ...