Warining !

if you have Android 9 ( CO2-3xxx ), Android 10 ( CO2-4xxx ) firmware
then you can not downgrade to Android 8 ( CO2-0xxx ) firnmware by 
simple flashing. Instead, you need to flash first 
CO2_Downgrade_Android_P.NBx.zip

if you bricked phone by direct downgrade, then use Bootfix_CO2.zip


Notice !

if you have Android 9 ( CO2-3xxx ), Android 10 ( CO2-4xxx ) firmware
then in order to use SERVICE function you need to flash
CO2-3220-0-00WW-B01_9.0.NBx.zip

after this, you can use SERVICE function.