Why does the ac smell like dirty feet when I first turn it on?
Because you are not wearing shoes.
Some water from the condensor is remaining in the AC system when it’s off and mold or mildew is starting to grow.
Perhaps the drain is slightly clogged and not allowing all of the water to drain out.
Some cars with cabin filters will smell like that if the fitter has not been changed recently. Also be sure to clean out the area under the grill just in front of the windscreen. You can buy a spray to help out also. I recommend Lysol or better but more expensive, the stuff made just for car from the auto parts store.