How do they put vitamin D in milk?

<<< "Basically D-3 always is derived from an animal. The sunlight reaction that converts 7-dehydrocholesterol to vitamin D-3 is a 'pure' chemical reaction that occurs in your skin in certain cells." Marian Herbert of the Vitamin D Workshop, a division of the biochemistry department at the University of California, wrote: "The provitamin known as 7-dehydrocholesterol is extracted and isolated from the skins of mammals and purified." I have subsequently learned that Vitamin D-3 can come from four different sources: Pig skin, sheep skin, raw fish liver, and pig brains.
Most of the time, Vitamin D-3 is extracted from pig skin and sold to dairy processors. >>>
Source: -----or "The milk is passed under a series of ultraviolet lights to fortify it with Vitamin D." source: