Sourdough rises using organisms that were already present on flour and in the surrounding environment rather than a packet of cultured yeast. The starter is a managed ecosystem, and the practices around it are ways of keeping that ecosystem stable.
Two kinds of organism share the jar
A mature starter contains wild yeasts, which produce carbon dioxide that leavens the dough, and lactic acid bacteria, which produce acids that give the bread its flavor.
They coexist because they use different resources. The bacteria metabolize sugars the yeasts leave alone, and the acidic conditions they create suppress competitors that would otherwise take over.
That mutual arrangement is why a starter becomes more stable over time. Once the culture is established, the acidity itself defends the jar against contamination.
Feeding is population management
Refreshing a starter means discarding most of it and adding fresh flour and water, which resets the food supply and dilutes accumulated acid.
Without refreshment the organisms exhaust available sugars and the environment turns increasingly acidic, eventually inhibiting the yeasts that provide lift.
The discard is not a waste of good culture but a necessary reduction in population, since the remaining organisms need a food-to-culture ratio that lets them multiply.
Temperature selects which organisms dominate
Warmer conditions favor bacteria that produce acetic and lactic acid at different proportions than cooler ones, which shifts the flavor of the finished loaf.
Yeast activity also responds to temperature, so a warm kitchen produces faster rises and a cold one slows fermentation without stopping it.
Bakers use this deliberately. A long cold retard develops acid and flavor while limiting gas production, which is why refrigerated overnight doughs taste more pronounced.
Gluten holds the gas that yeast makes
Leavening depends on two separate things: producing carbon dioxide and trapping it. The starter does the first, and the dough's gluten network does the second.
Mixing and folding align gluten proteins into a structure elastic enough to stretch around expanding bubbles without tearing.
Acidity affects that structure, strengthening it up to a point and weakening it beyond. Over-fermented dough spreads instead of rising because the network has degraded.
The culture reflects its handling more than its origin
Starters are often described as regional, but a culture adapts within weeks to the flour it is fed and the schedule it is kept on.
A starter carried across the country will shift toward whatever organisms thrive in its new conditions, so continuity of character comes from continuity of practice.
Which is why two bakers sharing one starter get different bread. The jar supplies the organisms, and the feeding schedule, flour and kitchen temperature supply everything else.