there is an older and traditional method,
the water mixing method is used for packaged buttermilk mainly cause they separate fat from the milk in an early phase.
churn the curd, butter will separate at top which will later be used to make ghee in india, rest is chaas or mattha traditionally which contains negligible fat and a good drink for summers.
I know the traditional one too. My grandma use to do it. But its really time taking and these days nobody have enough time and such equipments, hope you know for which equipments i am talking about. Thats why i put a simple one method.