Homebrewer here, they generally don’t die, they just go dormant. It’s quite normal to harvest yeast at the end of fermentation, to the point where many breweries do so to save money. You can even use the harvested yeast to make bread with (which is how bread used to be made)
If you’re going to make anything from milk that requires bacterial cultures and the conditions under which they will grow, you absolutely do not want whatever random cultures that are in a raw product. You start clean and add the cultures you want to propagate. Source: ferments things at home