Chapter 1, page 11 This code is basic but allows the reader to gain insight into key elements for several other projects in this book. It combines two pieces of code that are listed in the examples ...

If you include a library file through the menu, and it’s not part of the core Arduino libraries, the IDE locates its source code and compiles and links it in to the core library for you.

Open up the ESP32-specific directory that you cloned from GitHub. The usual *.cpp files provide the standard Arduino core functionality.