There are two main reasons why wild yeast bread, of which sourdough is a type, will taste different than the sandwich bread you buy in the store or make in a bread maker. The first is the fact that you’re utilizing wild yeast, instead of commercial yeast when making wild yeast bread. In many of the same ways as a wild boar will taste different than a farmed pig, wild yeast has a different flavor than commercial yeast. Commercial yeast has been bred to have the same flavor every time. However, wild yeast has a lot of biological diversity, so it creates a more complex yeast flavor. Part of the biological diversity is the fact that different strains of bread yeast in the air varies by location, so a wild yeast bread made in one location will taste different than bread made elsewhere. Additionally, the manner in which you feed it (e.g. how often and how much) will also dictate the flavor imparted by the yeast.
The second reason is because fermentation is part of the process. Commercial yeast happens so quickly that no, or at least a very minute amount of, fermentation takes place. Without fermentation, the second microbial component of wild yeast breads never proliferate in commercial yeast breads, Lactobacillus. Lactobacillus is the microbe that creates the sour flavor. It is the same type of bacteria that you find in yogurt and sauerkraut. The specific one in wild yeast or sourdough breads is Lactobacillus sanfranciscensis. This specific strain of Lactobacillus is only present in bread wild yeast cultures and exists no where else. The yeast creates an environment that is hospitable to Lactobacillus, but not to other competing bacteria. It also feeds on different sugars than the bacteria. Since the yeast first needs to make the starter hospitable to Lactobacillus, the bacteria takes at least 12 hours to start proliferating in a wild yeast bread culture. Therefore, it never takes hold in commercial yeast breads that only take two hours or less to rise.
Now where does bread yeast and Lactobacillus come from? Well, everywhere. It is in the flour, on your hands, and in the air. That’s why just flour and water will produce a starter. The yeast is already here and ready to proliferate in wild yeast starters. These microbes only live in bread yeast cultures, so they are just waiting everywhere to find the right environment and proliferate. If you think about it, us making wild yeast breads is the only way they can survive, so we have a responsibility to make wild yeast and not make these species endangered!