Op-amps are made out of multi-transistor circuits. If we were to go through about another three or four lectures on multi-transistor circuits, we would cover how the various multi-transistor components of an op-amp (current mirrors, differential amplifiers) work. If this were a two-semester course, we'd be covering this, but unfortunately we are going to skip this material so that we can spend some time on digital electronics.
This Wikipedia article has a schematic of the 741 op-amp, which has a bunch of bipolar transistors in it.