The grit to be applied to the sandpaper is poured from bags (that are purchased from outside sources) into a hopper.
There are various types of grit used in different types of sandpaper. For instance, many hand sandpapers are made using garnet or aluminum oxide, while many abrasive papers used for power sanders are manufactured using black zirconium (which is basically the same material that cubic zirconium faux-diamonds are made from).
Surprisingly, this sand grit in the hopper feels far smoother than it does on the sandpaper. The above picture is of some 120-grit abrasive, but it literally feels almost silky in the raw form.