Short answers, pulled from the story.
Filmjölk is a traditional Swedish fermented milk product made by culturing pasteurised cow's milk with Lactococcus lactis and Leuconostoc mesenteroides bacteria. The bacteria convert lactose into lactic acid, which thickens the milk and gives it a mildly sour, slightly buttery taste. It has a shelf life of around 10-14 days under refrigeration.
Filmjölk was introduced to the Swedish consumer market in 1931 by the dairy cooperative Arla. That first product was unflavoured and contained 3% milkfat.
Classic variants include regular filmjölk (2.5-3% milkfat, introduced 1931), långfil (a ropy, elastic type introduced in 1965), lättfil (0.5% milkfat, introduced 1967), and mellanfil (1.5% milkfat, introduced 1990). Probiotic variants include A-fil, Verum Hälsofil, ProViva, and Onaka fil, among others.
Filmjölk is better tolerated by people who are lactose intolerant than unfermented milk, because the bacteria metabolize much of the lactose into lactic acid during fermentation. Low-lactose variants treated with lactase enzyme are also available.
Tätmjölk is a variant of filmjölk made by rubbing the inside of a container with leaves of carnivorous plants, either sundew or butterwort, before adding lukewarm milk. Carl von Linné described a recipe for it in Flora Lapponica in 1737. Unlike standard filmjölk, no bacterial starter is added; the thickening mechanism involves protein-degrading enzymes from the plants, though the process is not fully understood.
Verum Hälsofil is a probiotic filmjölk produced by Norrmejerier that contains Lactococcus lactis L1A, a patented bacterial strain originating from a långfil culture taken from a farm in Västerbotten. In 1998, the Swedish Medical Products Agency (Läkemedelsverket) approved it as a natural medical product (naturläkemedel) after studies showed it has a positive effect on the immune and digestive system.