How whey is made

Source: https://www.youtube.com/watch?v=LeUZmojH7p8

1. Start with Milk, homogenize, pasteurize. It is 87% water, 5% lactose, 3.5% fat, 3.5% protein of which 80% is casine and 20% whey. So there is 0.7% whey in milk. Use around 10l milk for 1kg cheese.
2. Whey is a byproduct from making cheese which is made from the casine protein. You need to sperate that caseine from the rest of the milk. Add acid or bacteria that eat lactose and secret acid. Curdling happens and the milk gets firm as the casine separates from the rest, including the whey protein which stays in the water.
3. Now it is one big curd with all the other milk components trapped inside. Cut it in small pieces an press it. What comes out are the curds for the cheese and whey liquid.
4. The whey liquid contains protein, lactose and fat. 
5. Filter out the fat, lactose and minerals. Then dry it. You get whey

In short: Milk -> Filter out casine via curds -> filter out fat, minerals and lactose -> dry it -> whey powder.