Is buttermilk another term for sour milk or some part of sour milk? Is buttermilk another term for sour milk or some part of sour milk?
https://en.wikipedia.org/wiki/Buttermilk says:

Originally, buttermilk referred to the liquid left over from churning butter from cultured or fermented cream. Traditionally, before the advent of homogenization, the milk was left to sit for a period of time to allow the cream and milk to separate. During this time, naturally occurring lactic acid-producing bacteria in the milk fermented it. This facilitates the butter churning process, since fat from cream with a lower pH coalesces more readily than that of fresh cream. The acidic environment also helps prevent potentially harmful microorganisms from growing, increasing shelf-life.[3]
Buttermilk is the byproduct of butter making.
Butter is made by agitating cream (-&gt; the fatty part of milk) resulting in clumps of fat and a milky white liquid that contains nearly no fat and some protein. If the cream was soured before (either by aging or by inoculating the cream with lactobacillae), the buttermilk will also be sour. If the butter was made from unfermented cream, the buttermilk will be mild.
Uncultured buttermilk is rarely sold, even if butter is made commercially with regular cream, the resulting buttermilk is soured afterwards.
As buttermilk is made from a fraction of whole milk (the cream), you could say it’s a part of soured milk. I outlined the process of butter making in this answer, that should also help understanding buttermilk.