op amp vs transistor
The transistor doesn't have a voltage gain,it has a transconductance i.e it varies its current according to the applied voltage .OpAmps are voltage amplifiers that may be of cascaded single stage amplifiers .When you cascade amplifiers, the total gain will be the multiplication of the gains of all stages,thus leading to a much higher gain than a single stage amplifier .Also, some topologies can used to increase gain like cascode, gain boosting...etc .You may want to refer to "Analog CMOS Integrated Circuits Design "By Razavi chapter 9 to see some topologies of the OTA .