Is sour milk, soured milk, and milk that has gone sour, all the exact same thing? Related to this question: Is buttermilk another term for sour milk or some part of sour milk? and especially this answer to it: https://cooking.stackexchange.com/a/115571/93811 and two of the comments appended to that answer, specifically:
"Is soured milk the same as sour milk as in, 'I kept this milk in the fridge too long after opening, and now it is sour, although it was okay yesterday, and so I need to chuck it out and buy some more'. – Matthew Christopher Bartsh yesterday
@MatthewChristopherBartsh Not exactly. “Kept in the fridge and something grew” is the problem - you don’t know what grew. Way back before commercial milk production, there was a good(-ish) change that something desirable grew - souring meant “let stand, it’ll be thick by tomorrow” or it meant “add some of the existing product and let the microorganisms from that multiply and do their thing”. It’s too complicated for a comment to explain why exactly the former is less likely to work today than back in our (great...)grandparents’ time. – Stephie♦