It isn't a dumb question. The problem is that the answer could vary depending on the plant, so there is no "one size fits all" kind of answer. With bermuda grass, for example, you can get new grass plants from seeds, roots or pieces of stolon.

We live out in the country, surrounded by thousands of acres of grassland interspersed with woodland, so no matter how much grass I remove, a billion seeds constantly fly in and wash in nonstop from every direction. I'm resigned to always having to deal with a certain amount of weedy grasses. I just try to pull them out of the garden while they are small and easy to remove.