Hardware/Software layer of an android device
Normally, A blocked bootloader means that the bootloader will verify the signature of the installed OS / recovery to be booted. And recovery does not allow you to install a unsigned OS. If you unlock the bootloader, you can "flash" any firmware and the bootloader wont verify the signature. Also, you can now flash a custom recovery that will be able to install from unverified/unsigned sources. Your SIM can be locked at the modem level, bootloader level or at the OS level. It is typically done at modem or bootloader level though.

To unlock a bootloader, you either flash a modified bootloader or switch a flag (as done in nexus devices). root on phone is more or less like sudo.

Android OS firmware may refer to the Android OS (i.e. system, bin, etc. partitions) while Phone firmware is the OS + bootloader + modem and other require bits that are not the part of OS. Things like baseband processor OS, radio drivers,etc are also updated but are not part of android OS.