Here you can see a concept I did few years ago that can be used to answer your question. Usually when I paint big structures I tend to use simple shapes that can be read very easily. I then start to render and add detail to make the shapes look more complex. I also add elements such as pipes, panels or any sort of prop to these structures, which makes the surface more visually interesting.
To give a sense of a 3D structure, I overlap the elements to try and create a deep and interesting composition. Scale is also a nice thing to play with: put something easily recognisable that gives the viewer a sense of scale in the image.
In this case I put a large mech and tiny human figures working around it to show that the scale of this structure is massive. Try to find interesting contrasts between all those ideas and you can see that you don’t need to be an architect to create an idea of crazy structures. Of course, to apply all these tips you need to know the basics of perspective and lighting to begin with, but that should be a given for any subject.
Words: Francesco Lorenzetti