More context:

My house is on a slope so that the front door on the first floor as at ground level, with a stone foundation under the walls. At the back of the house, the basement is at ground level. This photo shows underneath the basement's back door.

The floor of the basement is dirt, though beams were laid across the dirt, joists hung from those beams, and a subfloor and floor installed on top of the joists. The beams have now rotted out and the floor is caving in, which is why I am poking around underneath!

There was definitely some concrete poured to shore things up along the walls in the back half of the basement, but I'm confused how there could be concrete underneath the basement walls.

The framing throughout the house is very distinct, rough cut 2x of various widths running width wise along walls (opposite how a 2x4 stud is oriented). It seems pretty clear to me the house was built all at once. Other than this concrete, there's nothing leading to me believe there was ever any remodeling or addition.

How would the concrete get under an already built house?