Kefir
This is how I make kefir, a fermented dairy product similar which is similar to a dairy-based kombucha. Colonies of microbes called "grains" are submerged into milk, turning it into kefir which is then separated from the grains with a strainer.
Prepared kefir is often drank as a probiotic beverage, but it can also be separated strained with cheesecloth to make a cheese called labneh, which is similar to Greek yogurt and can be used in its place in most recipes. The whey separated in this process is full of yeast and bacteria, which can then start other ferments like sourdough, wine, and creme fraiche.