You will really need to spend some time with the data sheet for the 4809.
There are two modes(free running and single conversion), two resolutions(8/10 bit) and interactions with Vref.
If you just want to modify the standard core for reading faster than 125KHz
check out this thread on how to set PRESCALER in ADC0.CTRLC
https://forum.arduino.cc/t/adc-register-nano-every/860146/12