think of the starter relay as an electrical switch (location to me is unknown) that draws very little currant with very small wires when the relay closes then it carries a larger load with bigger wires to the starter solenoid bigger contacts and the plunger make contact with the starter contacts which puts full battery voltage to the starter.
the starter solenoid is on top of the starter.
the whole prosses starts with minimum volts and end in maximum battery output.
hth