ESP8266 Internals

I'm using the NodeMCU 1.0 (ESP-12E Module) board as my first venture into using the Arduino platform, and I would like to learn more about the internals of the Arduino software that stands between my programs and the C++ compiler. Can anyone tell me where I should look for this kind of information?

For example: I'm using the "WRITE_PERI_REG" function extensively in my code, and would like to understand how it is implemented. Can anyone point me to where I would find the source code that implements this function? All clues appreciated!

Espressive uses the Arduino IDE, the device specific code is supplied by them.
I doubt that it is open source.

I think this is a good place to start https://www.espressif.com/en/products/hardware/esp8266ex/resources.

Thank you Whandall. I'll check it out.
Nice cat!