A discrete op-amp is made from individual (discrete) transistors, diodes, resistors, capacitors, and, occasionally, inductors. These components are brought together on a circuit board or substrate to create the final circuit. Each diverse component is fabricated on a manufacturing line that is fully optimized for that specific part. Therefore, each component is the very best it can be. Low-noise input transistors are fully optimized for their unique requirements. High-power output transistors are fully optimized for their unique and very different requirements. Precision resistors come from manufacturing lines that are dedicated to making precision resistors. Capacitors come from optimized capacitor lines. Only after these fully optimized components are fabricated are they brought together ona circuit board or substrate. - J Hardy