Google can force OEMs implement non-unlockable secure boot.
Fairphone wanted to give users full access on the Fairphone 2, but were contractually disallowed if they wanted to also ship the Android Market, Google Maps, etc., which users can't otherwise install themselves so it was essential to pre-install for a normal user experience. That's why they made two OSes for that phone: a googleful one and a free OS based on AOSP that you can install if you don't want Google (https://code.fairphone.com/projects/fairphone-2/fairphone-op...). Nowadays they let the /e/ Foundation do that work with e/OS. They're supportive of it but apparently don't have the internal manpower to continue making and supporting an extra distribution
Not sure why you're calling this non-unlockable. Everything is unlockable with enough money.
Maybe we use some hardware-level trick to get to some protected firmware initially to reverse engineer it, but almost universally it's what reads the state of the fuses (or something after it) that actually gets exploited. That's changing, too, but in general very slowly and at at the pace of hardware manufacturers learning how to make software (aka, glacial with a few notable exceptions).