My google-fu is amazing:
https://www.quora.com/How-is-2-fat-milk-made
"Using a piece of equipment conveniently known as a separator (think centrifuge) a milk processing plant can separate the milk fat (MF) and solids non fat (SNF) from the water. Milk is on average 3.5% MF, ~9% SNF and the remaining ~87% is water. The plant operator can then reduce the fat content to whatever level necessary. In California protein and calcium are added back to the milk so the Total Solids content is close to the same as before removing fat. By re-adding solids to the milk the taste is improved and had a less watery flavor."
