OK I'll give it a shot.
My understanding is that the grit stays in the gizzard, until it is worn down itself too small to be useful, then it is passed along and out. Chickens need a constant supply as it is pooped out with each poop, but is is more like dust or sand by that point. If chickens free range with access to small stones, sand etc they will get enough themselves. If they don't get it themselves then it needs to be supplied. And chicks should get grit as soon as they start eating anything besides starter.
Imp