Arduino magic? Compiler option? #ifdef #include #endif

@CodingBadly
Sorry, but I don't understand. Is this somehow related to my original question or just a side note?

If related: the #define AVR_ATmega328, IMHO, has nothing to do with my problem.
I could have posted this sketch and the original problem persists:

#define MYGUARD12 

#ifdef MYGUARD12
#include <RF24.h>
#elif MYGUARD14
#include <RF24_STM32.h>
#endif /* ifdef __AVR_ATmega32 */


int main() {
  RF24 radio(9,10);
}

Thanks & best