When I'm looking for such kind of information I usually use the GitHub searchbar into the Samd core. Keywords like "ENABLE" and "CTRLA" could be some good try.
To reduce the power consumption a lot, disabling some peripheral is not enough : you have to put the microcontroller into sleep mode, and wake up sometimes to do what you need.
You could try something like this :
void setup()
{
//Be careful here : there is no way to wake up the board with this code,
//so we need a delay to allow uploading new code
//!!! DO NOT REMOVE THIS DELAY UNLESS YOU KNOW WHAT YOU'RE DOING!!!
delay(10000);
// Set sleep mode to deep sleep
SCB->SCR |= SCB_SCR_SLEEPDEEP_Msk;
//Disable USB port (to disconnect correctly from host
USB->DEVICE.CTRLA.reg &= ~USB_CTRLA_ENABLE;
//Enter sleep mode and wait for interrupt (WFI)
__WFI();
//seelping ZZZZZZZ....
//Re-enable USB (should never get there because no wakeup interrupt is configured)
USB->DEVICE.CTRLA.reg |= USB_CTRLA_ENABLE;
}
void loop()
{
}
If you set up the RTC to wakeup the microcontroller each 1s or 10 or whatever and then go back to sleep, you'll be able to save A LOT of power.