Another way to define "smoothness" is as "the absence of textural forms". The smoother something is, the smaller and sparser the textural forms upon its surface will be. If something were perfectly smooth, it would not have any irregularities to its surface, resulting in that surface being extremely reflective. There would simply not be any forms to interfere with the angle at which light rays bounce off it.

There are very few objects that are so perfectly reflective, but even that would technically be a texture - just one characterized by its absence of textural forms, and therefore not particularly useful for our texture analysis exercise. That said, most real surfaces will provide you some kind of textural forms if - as you noted yourself - you zoom in closely enough. Everything's got some kind of irregularity, but rather than thinking about the microscopic irregularities we see with something like steel, where the scratches and knicks are only there if you zoom in, it helps to think instead at the opposite end of the spectrum.

If you move far enough away, the earth itself can appear like a smooth blue marble floating in the void of space.

As to the rest of your question though, the absence of a textural form means the absence of a cast shadow. So drawing a "smooth" texture isn't useful for the exercise. You'd either have to explore that surface at a scale that does provide texture (assuming you could find proper high resolution reference for it), or likely a better bet would be just to find a texture whose textural forms are more obvious. No need to purposely make this exercise any harder than it needs to be.

Lastly, be sure to review this section from the texture notes - it serves as a reminder that we are not simply copying our references, or drawing them directly. The references serve only to show us what textural forms are present. The marks we use to draw them in our texture analysis gradient are based on how we arrange those forms along our flat surface.