What determines the gain of a closed-loop op-amp circuit?

Operational amplifiers possess an inherently massive open-loop gain. However, in practical closed-loop circuits, a portion of the output signal is deliberately routed back to the inverting input to stabilize the amplifier and control its behavior. The ratio of the components used in this external feedback network (typically resistors) entirely dictates the final, overall voltage gain of the circuit. Because the op-amp's internal gain is so large, its specific internal values drop out of the math, leaving the external feedback components in complete control of the gain.