Flour already has yeast on it by the wheat growing out there in the field. It is not radiated or bleached or otherwise killed when the grain is milled so all that is brought forward in the flour. So that just flour and water and and a few days ignored in a closed mason jar will make excellent sourdough starter because the organisms survived all of that processing. Try it, you'll be amazed. One time I raced two slurries, one I collected for three days on the balcony and the other was slurry made the exact same way, both put in two separate mason jars, the same amounts each jar, and the uncollected slurry won the race. Both were excellent starters, but the slow to get going had a distinctly different flavor. Denver flavor. I'm imagining the organisms having a war accounting for the delay. The prevailing organisms are what creates the distinct local flavor. I'm imagining all this.
But I insist, and other sourdough aficionados dispute this, that collecting more organisms for a few days at least will result in sourdough unique to your location. Even while the thing that makes your location unique is the wind shoving organisms into your wet bowl of slurry that originates from other states. Weird, huh? Still, my Denver sourdough is qualitatively different from starter collected this same way in Concord California, and directly from wheat grown in Nebraska. Organisms rise up from the ground, rain droplets form around them and are deposited into your slurry.The ground that those organisms rise from makes the difference as the terroir of wine and all other food.
Prove it. Google search. [rain forms around organisms] Pick a page. Any page.
Or, [psuedomonas syringae + rain] then open the image tab and scan for a diagram. I see ten weather diagrams and then quit scrolling.
I drew this just for you.