Some use pulse-width modulation (PWM) to produce multiple output modes -- basically at lower output the LED is turned on and off very rapidly. Some people find that they can see the flicker and it bothers them.

Other flashlights are current-controlled to produce multiple output modes -- this is more expensive but works better and doesn't have the flicker.

Buck/boost circuits, as I understand them, increase or reduce the voltage output of the power source. This is separate from PWM or current control circuits.