When you make butter, you start with cream and agitate it either in a butter churn or a mixer (unless you are a butter factory, then you have huge machines that do the same thing).
Buttermilk, by definition, is low fat.
The agitation causes the fat molecules to separate and stick together, forming butter. What's left is skim or low fat milk that becomes buttermilk either naturally or with the addition of enzymes to aid the process.
"If you want to make an apple pie from scratch, you must first create the universe." -Carl Sagan