It may help you in your quest, if you understand just what a relay is and why it is used.
A relay is just like a wall switch. It works by using a small current, to operate access to a large current.
Think of the wall switch as the small current component (the relay itself). On the wall switch, you use your hand to throw the switch closed and then the house current rushes through it to power the light for example, but in a relay switch system, the low power side is the same as your hand, it opens and closes the main power circuit.
Two or three (sometimes), of the terminals on a relay, act as the light switch.
When you energize the relay by pushing a starter button for example, the 12V low amperage relay closes the contacts on the relay, which in turn, close the circuit to a 12V high amperage circuit, like the one powering the starter.
Hence, you are using a low power circuit to control a larger power circuit.
So in theory most times, the low power side controls the relay itself.
The high power side is controlled by the opening and closing of the relay.
Does this help your understanding somewhat?