[Android Q&A, Help & Troubleshooting] zhelan:
Found in init.usb.rc -> there is a reference to "debug.rootbackdoor=1"

# Copyright (C) 2012 The Android Open Source Project
#
# USB configuration common for all android devices
#

on post-fs-data
chown system system /sys/class/android_usb/android0/f_mass_storage/lun/file
chmod 0660 /sys/class/android_usb/android0/f_mass_storage/lun/file
chown system system /sys/class/android_usb/android0/f_rndis/ethaddr
chmod 0660 /sys/class/android_usb/android0/f_rndis/ethaddr
mkdir /data/misc/adb 02750 system shell
mkdir /data/adb 0700 root root

# adbd is controlled via property triggers in init.<platform>.usb.rc
service adbd /system/bin/adbd --root_seclabel=u:r:su:s0
class core
socket adbd seqpacket 660 system system
disabled
seclabel u:r:adbd:s0

### BSP: for_root @{
on property:debug.rootbackdoor=1
restart adbd
### BSP: for_root @}

###BSP:for usb backdoor start @{
on property:vold.usb.backdoor=charging,adb
setprop sys.usb.config ${vold.usb.backdoor}

on property:vold.usb.backdoor=test,diag,adb
setprop sys.usb.config ${vold.usb.backdoor}

on property:vold.usb.backdoor=none
setprop sys.usb.config ${vold.usb.backdoor}
###BSP:for usb backdoor end @}
on boot
setprop sys.usb.configfs 0

# Used to disable USB when switching states
on property:sys.usb.config=none && property:sys.usb.configfs=0
stop adbd
write /sys/class/android_usb/android0/enable 0
write /sys/class/android_usb/android0/bDeviceClass 0
setprop sys.usb.state ${sys.usb.config}

# adb only USB configuration
# This is the fallback configuration if the
# USB manager fails to set a standard configuration
on property:sys.usb.config=adb && property:sys.usb.configfs=0
write /sys/class/android_usb/android0/enable 0
write /sys/class/android_usb/android0/idVendor 18d1
write /sys/class/android_usb/android0/idProduct 4EE7
write /sys/class/android_usb/android0/functions ${sys.usb.config}
write /sys/class/android_usb/android0/enable 1
start adbd
setprop sys.usb.state ${sys.usb.config}

# USB accessory configuration
on property:sys.usb.config=accessory && property:sys.usb.configfs=0
write /sys/class/android_usb/android0/enable 0
write /sys/class/android_usb/android0/idVendor 18d1
write /sys/class/android_usb/android0/idProduct 2d00
write /sys/class/android_usb/android0/functions ${sys.usb.config}
write /sys/class/android_usb/android0/enable 1
setprop sys.usb.state ${sys.usb.config}

# USB accessory configuration, with adb
on property:sys.usb.config=accessory,adb && property:sys.usb.configfs=0
write /sys/class/android_usb/android0/enable 0
write /sys/class/android_usb/android0/idVendor 18d1
write /sys/class/android_usb/android0/idProduct 2d01
write /sys/class/android_usb/android0/functions ${sys.usb.config}
write /sys/class/android_usb/android0/enable 1
start adbd
setprop sys.usb.state ${sys.usb.config}

# audio accessory configuration
on property:sys.usb.config=audio_source && property:sys.usb.configfs=0
write /sys/class/android_usb/android0/enable 0
write /sys/class/android_usb/android0/idVendor 18d1
write /sys/class/android_usb/android0/idProduct 2d02
write /sys/class/android_usb/android0/functions ${sys.usb.config}
write /sys/class/android_usb/android0/enable 1
setprop sys.usb.state ${sys.usb.config}

# audio accessory configuration, with adb
on property:sys.usb.config=audio_source,adb && property:sys.usb.configfs=0
write /sys/class/android_usb/android0/enable 0
write /sys/class/android_usb/android0/idVendor 18d1
write /sys/class/android_usb/android0/idProduct 2d03
write /sys/class/android_usb/android0/functions ${sys.usb.config}
write /sys/class/android_usb/android0/enable 1
start adbd
setprop sys.usb.state ${sys.usb.config}

# USB and audio accessory configuration
on property:sys.usb.config=accessory,audio_source && property:sys.usb.configfs=0
write /sys/class/android_usb/android0/enable 0
write /sys/class/android_usb/android0/idVendor 18d1
write /sys/class/android_usb/android0/idProduct 2d04
write /sys/class/android_usb/android0/functions ${sys.usb.config}
write /sys/class/android_usb/android0/enable 1
setprop sys.usb.state ${sys.usb.config}

# USB and audio accessory configuration, with adb
on property:sys.usb.config=accessory,audio_source,adb && property:sys.usb.configfs=0
write /sys/class/android_usb/android0/enable 0
write /sys/class/android_usb/android0/idVendor 18d1
write /sys/class/android_usb/android0/idProduct 2d05
write /sys/class/android_usb/android0/functions ${sys.usb.config}
write /sys/class/android_usb/android0/enable 1
start adbd
setprop sys.usb.state ${sys.usb.config}

# Used to set USB configuration at boot and to switch the configuration
# when changing the default configuration
on boot && property:persist.sys.usb.config=*
setprop sys.usb.config ${persist.sys.usb.config}

#
# USB type C
#

# USB mode changes
on property:sys.usb.typec.mode=dfp
write /sys/class/dual_role_usb/otg_default/mode ${sys.usb.typec.mode}
setprop sys.usb.typec.state ${sys.usb.typec.mode}

on property:sys.usb.typec.mode=ufp
write /sys/class/dual_role_usb/otg_default/mode ${sys.usb.typec.mode}
setprop sys.usb.typec.state ${sys.usb.typec.mode}

# USB data role changes
on property:sys.usb.typec.data_role=device
write /sys/class/dual_role_usb/otg_default/data_role ${sys.usb.typec.data_role}
setprop sys.usb.typec.state ${sys.usb.typec.data_role}

on property:sys.usb.typec.data_role=host
write /sys/class/dual_role_usb/otg_default/data_role ${sys.usb.typec.data_role}
setprop sys.usb.typec.state ${sys.usb.typec.data_role}

# USB power role changes
on property:sys.usb.typec.power_role=source
write /sys/class/dual_role_usb/otg_default/power_role ${sys.usb.typec.power_role}
setprop sys.usb.typec.state ${sys.usb.typec.power_role}

on property:sys.usb.typec.power_role=sink
write /sys/class/dual_role_usb/otg_default/power_role ${sys.usb.typec.power_role}
setprop sys.usb.typec.state ${sys.usb.typec.power_role}