They actually were related but probably before you were born. Butter before pasteurization was originally made from cultured cream, European butter still is. That left the original non fat acidic buttermilk. Nowadays American butter is sweet cream butter so the leftovers are different. To get the acidic product generally low fat milk is cultured, usually there is live culture left and you can use this faux buttermilk to culture more milk or cream to make more buttermilk or creme fraiche. I've got some buttermilk I cultured in my fridge now, I wouldn't even call it a liquid though it's thinner than yogurt. I left this batch out a bit long.