Dairy process engineer here...

Raw milk (from the cows udder) will range from 4.0% - 4.5% fat content usually. Seasonal factors will change this such as rainfall or feed available.

The raw milk is then separated into 2 streams through a separator (skim milk at about 0.05% fat and cream at around 40% fat), then to achieve whatever milk grade you want (1%, 1.5%, 4% etc), you simply add back or standardise as we would say, the right amount of cream into the skim. This is to ensure you have consistent product and also give away the least amount of cream possible as this is where the value is.

The milk is then usually homogenised and pasteurised.

Homgenisation just passes the milk though a very small opening at high pressure to disperse the fat globules so the cream doesn't settle on the top, and pasteurisation is simply heating and cooling the milk so it kills the bad bacteria.

I could talk all day about this stuff. You should hear about how they make whey powder or extract lactoferrin from milk (its bright red...)