I once had the romantic idea of making beer from a 200 year old recipe. Didn't get too far with it, but I recall one of the pieces of advice being not to let your horse stop walking while it's stomping the grain from the chaff, because it will urinate and ruin your ingredients. Also no mention of yeast - I guess you just took it on faith that fermentation would start if you kept it wet and warm.
If you are using whole grains there is already natural yeast on it. Yeast is also pretty much everywhere. Nutrient rich solution left open will ferment. It just might not be a super powered brewing yeast.
Scandinavian brewers had a "magic stick" used to capture and store yeast until the next batch.
Brewing, like baking, was a regular occurrence in a household. It was done in the same place, with the same equipment, as last time. The yeast culture from the previous batch was likely a given ingredient.
imagine doing this when there was little notion of things like "pH" and "temperature control was literally a matter of feel.