Each driver fills out a card that is kept by the 'Milk Men' that shows what type of milk they want. And then when the checkered flag falls, the bottle is filled with that type from gallon jugs.
Only one person knows what dairy the milk is actually from, and it's never revealed so the dairy can't use it for promotional purposes.
And this is one of 4 bottles. The winning team owner and winning chief mechanic also get one. The fourth one is a second driver's one that is given to ABC TV to shoot the pre-race intro with.
Here's the wall in my office showing the winners bottles I've done. Started with Hornish, and this picture was before Dario's last year. My dad handled it before then.