@Strixx the MySensors library does not have a pin reference.
The pins are defined by the board definitions.
See here for the NodeMCU Dn pins and BUILTIN_LED
See here for the A0 pin.
These definitions are just aliases. So A0 is an alias for PIN_A0 which is an alias for 17. You can use any alias you like, they will all result in the same code. I like the PIN_A0 notation because I think it is very clear that you mean the pin called A0 and that makes the code easier to read. But you can write 17 in your code and the code will work the same way as with PIN_A0.
For details on how the esp8266 works, I recommend Kolban's book.