Why An op-amp is an analog building block: it amplifies the difference between its two inputs (inverting and non-inverting) with very high open-loop gain, often 100,000 or more, and it is direct coupled so it works down to DC. Its input impedance is very high, so it draws almost no current from the source, and its output impedance is very low, so it can drive a load without the gain changing. Those ideal traits are what let external feedback components alone set the circuit's gain and response.
Watch out The choice calling it a digital audio amplifier gets one thing right, that external components set the characteristics, but an op-amp is an analog device, not digital, and it is not limited to audio.
Op-amp ideal: infinite gain, infinite input Z, zero output Z, differential and DC coupled.