Inside the concrete walls of the stairs, steel plates attached to steel rods (called imbeds) are woven into the rebar patterns. These imbeds give the steel beams a place to attach to and “hang off” the concrete wall. They weld perpendicular plates (called knife plates) to the imbeds and bolt the beams through.
This particular imbed is for the gym floor beam the attachment is so crowded that there is barely 3/4″ between bars. We can all sleep well with engineering like this.