How is 240v derived from 120/208v feed
It isn't.
120/240V is a 240V single phase supply, center tapped to produce a &quot;split phase&quot; with 120V between either side to neutral, and 240V between the two hot lines.
120/208V is two phases from a three-phase supply.  There is 120V from either phase to neutral, and 208V between the phases.