Kefir used to be an exotic drink, but it's now available in most supermarkets, at least where I shop. It was popularized in the US by the Lifeway company, beginning in 1986, but now is made by a number of brands. Kefir's origins are in Russia and several other countries in the East (for more history see the Wikipedia article).
The yeasts and bacteria in the cultures for each type of fermented dairy product are related to one-another, but not all the same. The processes for making them, such as the temperature and length of time for fermentation, also differ among the products, whether at home or in an industrial setting. Like sourdough starters for fermenting bread, you can buy commercial starters for home-made cultured milk products.
SCOBY is the name for these starter cultures. I thought it was a cute pet name but it's actually an acronym: "symbiotic culture of bacteria and yeast." When you make yogurt or sour cream, you can save out some of the product to use as the starter for the next batch. Kefir is a little different.
Kefir "grains" shown with scale.Source: Wikipedia.
"Kefir is a complex fermented dairy product created through the symbiotic fermentation of milk by lactic acid bacteria and yeasts contained within an exopolysaccharide and protein complex called a kefir grain. ... The beverage itself typically has a slightly viscous texture with tart and acidic flavor, low levels of alcohol, and in some cases slight carbonation. Kefir is traditionally made with cow’s milk but it can be made with milk from other sources such as goat, sheep, buffalo, or soy milk. One of the features that distinguish kefir from many other fermented dairy products is the requirement for the presence of a kefir grain in fermentation and the presence and importance of a large population of yeasts. The aforementioned kefir grains are microbially derived protein and polysaccharide matrices that contain a community of bacterial and fungal species that are essential to kefir fermentation. Traditionally, fermentation was initiated through the addition of kefir grains, which originally formed during the fermentation of milk, to unfermented milk in a sheep or goat skin bag. Commercial, industrial-scale production rarely utilizes kefir grains for fermentation, but rather uses starter cultures of microbes that have been isolated from kefir or kefir grains in order to provide more consistent products." (quote from: The Microbiota and Health Promoting Characteristics of the Fermented Beverage Kefir.)