You make this levain well before you mix the dough, so it has time to ferment, and eventually, it’s added to the dough to seed fermentation.
Remember: the levain is an off-shoot of your continually maintained sourdough starter and is likewise composed of bacteria and yeasts. You make this levain well before you mix the dough, so it has time to ferment, and eventually, it’s added to the dough to seed fermentation.