It is wild fermentation using a salt brine. It is the oldest and most fundamental form of fermentation. It just takes around 3% of a natural salt, some water, time, some kind of sugar or food for microbes to digest, and a sealed container. The container must be burped regularly or the pressure will build until it explodes in the worst cases. The lack of oxygen is key to stop most mold growth. Mold is the enemy. Microbes are you're drinking buddies. They are everywhere. Wild fermentation is random in results. There is no telling which microbe present will dominate the mix. Generally, it will be one of the ones that naturally live in and on the fruit. One way to make cool stuff is to mix fruits in ways where the microbes from one can use it to dominate another. Like toss a local apple in a food processor whole, let it go off in a container for a couple of days until it is very active, then toss this into a container of grape juice that is not active yet.