@kie-William some of the libs are pulled from external repos. An example is the SoftwareSerial in the case of the ESP8266. However, the ESP8266WebServer.cpp file is in this repo, in one of the provided libraries. If you can't find the file in your computer, but the ESP8266 examples build, then you're looking in the wrong place. Read the docs thoroughly, all you need is there. I certainly didn't need much more than that when I first started out, and the docs were more primitive back then.