kmk_firmware/docs/boot.md

There is a more detailed explanation in the circuit python docs, however there are some common use cases for your keyboard listed here

You can hide your device from showing up as a usb storage

storage.disable_usb_drive()

Make your keyboard work in bios

usb_hid.enable(boot_device=1)

Disable serial comms

# Equivalent to usb_cdc.enable(console=False, data=False)
usb_cdc.disable()

A fully working example, which disables usb storage, cdc and enables bios mode

import supervisor
import board
import digitalio
import storage
import usb_cdc
import usb_hid

# This is from the base kmk boot.py
supervisor.set_next_stack_limit(4096 + 4096)

# If this key is held during boot, don't run the code which hides the storage and disables serial
# To use another key just count its row and column and use those pins
# You can also use any other pins not already used in the matrix and make a button just for accesing your storage
col = digitalio.DigitalInOut(board.GP2)
row = digitalio.DigitalInOut(board.GP13)

# TODO: If your diode orientation is ROW2COL, then make row the output and col the input
col.switch_to_output(value=True)
row.switch_to_input(pull=digitalio.Pull.DOWN)

if not row.value:
    storage.disable_usb_drive()
    # Equivalent to usb_cdc.enable(console=False, data=False)
    usb_cdc.disable()
    usb_hid.enable(boot_device=1)

row.deinit()
col.deinit()