https://upload.wikimedia.org/wikipedia/commons/5/52/Buck_operating.svg
That is a schematic for just about the simplest SMPS, the buck regulator.
The capacitor value is not important as long as it is large enough. Bigger is always better.
The energy storage device that actually does the work is the inductor. The important thing to understand about the inductor is that the current in the inductor only changes relatively slowly. When the switch closes, current builds up in the inductor, but it only does so slowly, so the switch can open before the current is too big.
When the switch opens, the inductor current cannot change instantly. Initially, it is still at the maximum. The only place current can come from is through the diode. The current will fall, relatively slowly, but continue to supply the load, until the current reaches zero or the switch closes again.
That's about all there is to a buck regulator. The load is supplied some of the time from the supply, and some of the time from the diode, so the output current is more than the input current. The output voltage is, of course, always lower than the input voltage.
The rest is maths, and physical limits to what currents and voltage the components can stand, and imperfections because real components don't work quite like ideal ones.