Is buttermilk another term for sour milk or some part of sour milk?
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.