How does sourdough starter actually rise bread without any commercial yeast?

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A sourdough starter is genuinely a living culture of wild yeast and specific beneficial bacteria that naturally exist in flour itself, and in the surrounding air, rather than any single purpose bought commercial yeast strain. When you mix flour and water together and simply let the mixture sit, those naturally occurring wild yeasts and bacteria start actively feeding on the natural sugars present in the flour and begin multiplying steadily over time.

As that natural wild yeast population feeds and grows. It produces carbon dioxide gas as a normal metabolic byproduct, and that gas is exactly what actually gets trapped inside the developing gluten structure of a dough, causing it to rise, the exact same core basic mechanism commercial yeast uses, just from a genuinely different and much more varied naturally occurring source of yeast organisms.

The specific bacteria living alongside that wild yeast. Mainly various types of lactobacillus, produce lactic acid and acetic acid as their own separate metabolic byproducts, and those specific acids are precisely what gives true sourdough bread its distinctive tangy flavor, an actual real flavor commercial yeast alone genuinely cannot properly replicate on its own without that separate bacterial component present.

Regularly feeding a starter with fresh flour and water keeps that whole living culture properly balanced and genuinely active. Since the existing yeast and bacteria will otherwise eventually exhaust their available food supply and the whole culture can noticeably weaken or even die off entirely without that regular ongoing maintenance.

So the actual rise itself comes from real biological fermentation using naturally occurring wild organisms rather than any single purchased commercial strain. Which is exactly why every single sourdough starter genuinely tastes at least a little different depending on the specific flour used and the particular unique local environment it was actually originally cultivated in