Food industry: where does buttermilk go?
If it's made from 'sweet cream', and not soured milk (it's easier to churn soured milk, so this was typical in the old days), then what's left is skim milk ... although there might be an extra buttery taste to it.

Some of it's used to make powdered milk; I don't know if any is actually resold as skim milk.  It's possible that cultures are added to make 'buttermilk'.  And you might think 'there'd be way too much milk powder left over vs. the amount of butter made' ... but it's used in protein powders for body builders and infant formula.

... and it can be used for animal feed.  (and it's safer than other milk by-products, as it won't cause as much indigestion as the whey left over from cheese making (salty), or drained from greek yogurt (acidic).

Update: I was basing my answer on something that I saw years ago (How It's Made, Unwrapped, some other similar TV show), and some knowledge of uses of powdered milk.  However, in doing some additional research:


Skim milk is separated off before the butter making process.  It's done by centrifuge, so the input to the butter making process is higher fat content (and lower moisture) than what a home butter-maker would be working with.
Protein powders are typically made from whey powder, not non-fat milk powder
Much powdered milk gets shipped to developing countries, as it's cheaper to ship and store.  (less weight and volume, and doesn't need refrigeration) ... but much of that is full-fat powdered milk
Non-fat milk powder is used in a lot of baker products and processed foods, as it can serve as a thickener, increases protein content, and promotes browning of baked goods.  (and it can be lower cost, due to reduced shipping and storage costs)  So when you see 'milk' on an ingredient list, it might actually be powdered milk.


So, there's probably not as much left over as we'd expect based solely on knowledge of home butter churning and the amount of butter production.  And I was off on where all of the non-fat powdered milk gets used.