but for a thermo starter, you have to buy the yogurt? This is what is confusing me, because if they did it in the past, without all the labs, etc, how do I do this at home, starting from raw milk?
By heating past the lysis point of meso bacteria but not thermo bacteria. It happens naturally in farmstead scenarios by either leaving milk out in the full hot sun, which kills meso bacteria, or by heating in a copper vat and then retaining some of the whey to use for backslopping.
This was before the advent of microbiological techniques. After, as Sailor pointed out, it's just normal pure culture isolation of commonly used starters.
and am I right about the meso starter? and what about kefir grains? How do you get those?
Their origin is lost, but if you somehow had all the bacteria and yeasts together, they could form a grain. I suspect originally, milk had gone bad and was left, and then someone smelled it and it smelled like sour milk, so it was eaten. And in the end some grains were left over, and some superstition likely made a person add more milk to the grains. Well until the 1920s it was impossible to get kefir grains. They were obtained by this crazy way from the prince in the Caucassus involving a kidnapped Russian industrial beautiful spy (not even kidding).
My thought is this, lets say the economy takes a dive, no internet, or something like that, and I can't go online and purchase the starter packets. What then? I want to be able to do it at home...... are you understanding?
Then you culture raw milk and make whatever cheese you can. For thermo, you have to kill off the meso and it will form thermo culture. But honestly, like for yogurt, it's not really thermo that acidifies, it's strep thermophilus, which is a hybrid kind of bacteria. It's not like lactococcus because it tolerates higher temps, and it acidifies faster.
The other way that thermo cultures are isolated is again naturally. L bulgaricus, for example, can tolerate extremes of acid development. So when meso cells start to die at 4.5, bulgaricus will keep producing acid until 3.5. L helveticus isn't ever really created in a farmstead setting because initial concentrations of helveticus for common thermo cheeses is pretty low, just naturally ocurring or from a backslop. Helveticus grows and multiplies as the cheese ages.