All of the above are often referred to as either starters or sponges. It preserves the makeup of the leaven culture used at any particular bakery. Most bakeries now use more liquid leavens, and store them in the refrigerator when necessary. Levain is the French term for a sponge or soft dough that is being used to propagate a sourdough culture. It may incorporate all the water that will eventually be in the dough, or some portion in it.
Yeast are available with high resistance to freezing, for example.
Though most yeast packets contain some bacteria, there are not enough to produce the acid and the volatile organic compounds that give sourdough bread its flavor. Also, most people use so much commercial yeast that the bread tastes more of it than of wheat.