Installing the concrete on the basement floor is done by marking around the edge of the foundation a level line and then filling the basement with concrete up to the line, usually 3 1/2" to 4" up from the top of the footing.
This image shows the leveling of the base layer under the concrete slab. It is often done with gravel to provide good compaction and make it easier to get a level base which saves on concrete costs. Notice that around the plumbing pipes sand is used to prevent puncture of pipes by larger stones.
what else happens on this day
Install the flatwork in the garage and possibly the driveway, depends on the size of the concrete crew.