Here are the main steps in making whey protein isolate.
Whole milk -> Cheesemaking -> Cheese and Whey (whey protein, a lot of lactose, a lot of minerals)
Whey protein concentrate (25-80% whey protein, up to 55% lactose, 1-7% fat)
Whey protein isolate (>90% whey protein, negligible lactose, <1% fat)
Whole milk = 87% water and 13% solids
Milk solids = 27% protein, 37% lactose, 30% fat and 6% ash/minerals
Milk protein = 20% whey and 80% casein