These holes were probably made originally by small rodents such as mice or voles. Snakes do not create holes, but many species spend a large portion of their lives in and out of the burrows of rodents. Even the species we call burrowing snakes simply bury themselves in loose soil, they do not leave an opening at the surface.
All of the animals you have mentioned: snakes, yellow-jackets, frogs, and a dozen more use the burrows created by rodents (and a few other burrowing animals) for shelter. And yes, you could have an underground city of frogs down there. Amphibians that are active at night usually spend the day in the moist, cool habitat provided by an abandoned burrow. Go out there at night with a flashlight and be patient, you will see just how many critters are sharing your property.