On the right track but not quite! Every brush in art software uses an image (raster or auto-generated) to paint with, called a "brushtip". Usually, the brushtip only gives information about which bits are opaque, and which arent - the shape! However in Krita, theres another dimension you can define; value, or lightness.

So there's nothing being generated or created while drawing, its just that some very smart people have coded Krita for the "brushtips" to do more as a baseline.

Not every software works exactly the same ofcourse! This is just my beginner level understanding of it all, I hope that helps