Of course, this wasn’t my final code, but it shows how easy it was to write some portable code to work with the serial port.

In it, he shows an example of using all three ESP32 UARTs from an Arduino program. He calls the third port “secret” although that’s really a misnomer.

Before USB ports became standard on PCs, several devices connected through a serial port. They're obsolete in many ways, but they still see some use.