Quick question from a Noob - if the bootloader is locked, does it mean fastboot OEM unlock will fail?
The short answer is “yes,” but the exact syntax of the commands used can vary according to the manufacturer.
It is comparatively easy to unlock most Pixels, but I seem to recall that there are versions for some carriers in the U.S. that might remove the unlock functionality.
After backing up your phone, depending on the manufacturer, you might first have to run a command like “fastboot oem get_unlock_data” (Motorola) or “fastboot oem get_identifier_token” and submit the data you get from that command to your manufacturer to get an unlock token.
After doing that, OR if you have a handset from a manufacturer that makes it easy, you might find that an option toggle called something like “OEM unlocking” shows up in your Developer Mode settings in the handset’s Settings menu.
Doing these unlocks is not particularly difficult for the technically inclined, but it is important to do your research first and not to just type commands you find in some article without knowing the nuances of your particular device. They might erase your data, so backups are a good first step.
Note: even manufacturers who are good about allowing bootloader unlocks do have models or carrier-tailored versions that might not allow it, or that might require you to get an unlock token when the rest of their product line doesn’t.
For Android phones, one of the best resources is a site called XDA-Developers.
I hope that helps. Good luck!