Control Circuitry in the Buck Converter Design
The control circuitry is internal to the IC; on page 12 of the spec it's in the lower and center left of the block diagram.  It takes the feedback signal and uses it to adjust the switching frequency or duty cycle, depending on the chip.  It may also introduce other control or filtering techniques.
The loop can be considered a sampled system at the switching rate, which is why the answer suggests keeping the dominant pole of the control loop 5-10x slower for stability.