Ever since the best decade of all time, the 1970s, people have been able to buy amplifiers in all-in-one chips rather than having to make them out of lots of discrete transistors. This video shows you the basics of op amps and has an example circuit where you build an amplifier to take a tiny microphone signal and amplify it to be audible on earbuds. The circuit is based on the LM324 quad op amp IC.

The circuit diagram from the video: