Somewhat simplistic, but one way to think about a buck converter is an input chopper followed by low pass filter to remove the chopping ripple.
For example, if the input chopper is running at 50% duty cycle, the output of the L-C filter will be about half the input supply. This does not take into account resistive losses of the inductor and resistance of the input chopper devices. An open loop (no control feedback) can be made like this but there will be some voltage drop on output as it is loaded. A feedback correction would modified the chopper duty cycle to compensate for circuit losses to maintain a constant output voltage.