That is the source but it can be nice to access the actual files on your computer to be sure you are looking at the exact version in use or if you want to modify it. An easy way to find the source files for an Arduino library is:
- File > Examples > ESP8266WiFi > select any example
- Sketch > Show sketch folder - this will open the folder containing the example sketch
- Navigate up two folder levels, this will be the library root folder. In some libraries you may find the source files in the src subfolder, others will be in the root folder.