Nokia has no bootloader unlocking, except for the Nokia 8 and the have publicly stated they don't plan to add new devices to the unlock program.

Some older devices are supported through some weird unofficial sites where you have to send in your IMEI, which is obviously shady...

Also they don't update their kernel sources in a timely manner at all. Which makes developing custom ROMs WAY harder..

So sadly, Nokia sucks if you wan't custom ROMs, I ditched my Nokia 8 for a Poco F1 for this reason.