A few hundred thousand years ago, this garden was a silty river delta. Today, with the river long-gone, the silt remains and forms the native soil. Mulched regularly over 12 years, the native soil (the lighter yellowy clods) became the darker color through the work of earthworms slowly incorporating the mulch into the soil over time.
The transformed soil has tiny air pockets, organic content, greater nutrient content, and stays moister longer than the original, all without any effort on the gardener's part besides the tossing of the mulch. Sometimes, not even that.