I kind of did a long time ago, but I can only remember a little about it.
Basically, there's an app on Stock Pixels that checks if a device is carrier/OEM locked. That app (
com.google.android.apps.work.oobconfig) is pretty much the only app that has access to toggle whether the "carrier" allows the phone to be unlocked. What this app does is it checks a Firestore record for the phone it's installed on to see if the "carrier" allows the bootloader/SIM to be unlocked.
Some on a three letter website think all phones are locked in the factory and only unlock via this app. I don't know whether that's right or not, but it doesn't matter since it wouldn't be difficult for Google to add some stuff to their proprietary apps to check if the oobconfig app had successfully checked the phone's info before enabling the toggle for the user.
So when Google says they "can't" unlock a locked phone, that's complete BS. I'm sure customer service people don't have access, but Google can totally unlock phones remotely.