How do they enrich flour anyway?

I looked it up, and they tell me the “why” and the “when” for enriching flour, but not the “how”. So what is it?

Do they just grind up vitamins and add vitamin powder to the flour (in which case, if you shook a bag would the vitamins all end up at the bottom of the bag?) or do they somehow chemically bind the vitamins and minerals to the grains? Or what?

See this link and sources cited therein