The Arduino IDE uses #defines to define things like I/O pin numbers ... If you connect a jumper wire to pin D13 on the Arduino header, then connect it to an LED on a breadboard, it will blink ...

If you use the Arduino IDE to program the ESP32, you might be interested in [Andreas Spiess’] latest video (see below). In it, he shows an example of using all three ESP32 UARTs from an Arduino ...