Yogurt requires 2 bacterial strains to be called yogurt.
The 2 bacterial strains required to make yogurt:
Bacterial strain 1: Streptococcus Thermophilus
Ideal temperature range: 95-108°F / 35-42°C
This bacteria's main job is to acidify the milk from a pH of about 6.5 down to about a 4.6 pH.
Bacterial strain 2: Lactobacillus delbrueckii subspecies bulgaricus
Ideal temperature range 109-115°F / 43-46°C
This bacteria is activated once the milk is acidic enough. Its main job is to solidify the acidic milk into yogurt.
This solidification is accomplished by even further acidification of the milk. When casein in the milk is acidic enough for a long enough time, it will solidify (precipitate) into yogurt. This second bacteria is necessary to hold that 4.6 pH level that the first bacteria cannot hold. This is how yogurt is made.
The most common practice is to use a previous batch of yogurt to make an upcoming batch of yogurt. In factories where they have controlled conditions, they do not need a previous batch because they can isolate these two bacteria strains and inoculate this way. Keep in mind that it is impossible to make yogurt without a bacterial culture.
Please note that the Lactobacillus delbrueckii subsp. bulgaricus bacteria can technically be substituted for another similar lactic acid bacteria (aka LAB) that also solidifies casein, but this approach is much more difficult compared to simply using a previous batch of successfully made yogurt.
Using probiotics, L. reuteri (ie SuperGut protocols), or any other bacteria will result in a "fermented milk" and can make a delicious product, but technically speaking it cannot be called yogurt. These products are still fantasic, but this nomenclature distinction should be kept in mind for those interested.
How these two bacteria transform milk into yogurt
Watch my video below for more information on the microbiology and chemistry behind making yogurt.