AVR Freaks

I'm very new to programming SAM mcus so please bear with me. I'm using an Adafruit Metro M4 board.

I wrote some code in the Arduino IDE that I'd like to write the equivalent of in Atmel Studio by writing the registers directly.

void setup()
{
  PORT->Group[PORTA].PINCFG[21].bit.INEN = 1;  // Enable PA21 (pin 8) as an input
  PORT->Group[PORTA].PINCFG[21].bit.PULLEN = 1;
  PORT->Group[PORTA].DIRCLR.reg = PORT_PA21;
  PORT->Group[PORTA].OUTSET.reg = PORT_PA21; //set the pullup resistor
  attachInterrupt(digitalPinToInterrupt(8), fallISR, FALLING);
}

void loop()
{
  //do nothing
}

void fallISR()
{
  //if there was a falling edge on pin 8, do what's in this ISR
}

I have no clue how to write the equivalent of attachInterrupt() in Atmel studio. From what I've seen it's fairly complicated and requires a lot of lines of code. Definitely harder than 8 bit AVRs like I'm used to.
I thought the Arduino github documentation would help but it's farily confusing and seems to only work for the SAMD21?

https://github.com/arduino/ArduinoCore-samd/blob/master/cores/arduino/WInterrupts.c

When I attempt to copy and paste the code from this library several errors show up, for example when I copy and paste

NVIC_SetPriority(EIC_IRQn, 0);

I get the error "'EIC_IRQn' was not declared in this scope; did you mean 'ICM_IRQn'?"
I don't see anything about EIC_IRQn or ICM_IRQn in the SAMD51 datasheet so I'm not sure where to find what these things mean.

Same with "NVIC_SetPriority". I just have no clue where any of this stuff is coming from. Any help is greatly appreciated