I don't see what the issue is: googled esp8266 arduino core, got the Github page and then just drilled down to the NodeMCU variant's pins_arduino.h file at https://github.com/esp8266/Arduino/blob/master/variants/nodemcu/pins_arduino.h
And on my local drive it's at ~/.arduino15/packages/esp8266/hardware/esp8266/3.1.2/variants/nodemcu/pins_arduino.h Your copy will be wherever your O/S keeps the arduino15 directory.