OEM unlocking is not the same as carrier unlocking
Carrier unlocking is to allow you to use your phone with a different network e.g switching from Verizon to T-Mobile and putting a T-Mobile Sim in the Verizon phone. This can be done with both Android and iOS
OEM unlocking is an Android setting that enables the end user to modify the firmware of the device e.g unlocking the boot loader, installing custom boot images, ROMs and gaining root access.
AFAIK both T-Mobile and Verizon have custom Android settings that completely grey out the OEM unlock option in Android settings. There is no way around this as an end user, even if you had got customer support to carrier unlock your phone it would have made no difference, OEM unlocking would still be disabled.
AFAIK There is no way around this and the only solution is to never buy from either carrier.
I bought a refurbished Pixel a few years ago from the Netherlands and it ended up being originally from Verizon, I found a Verizon system app still preinstalled in settings and it was impossible it enable OEM unlocking.
Either buy directly from Google or buy second hand on the condition that you can check to make sure OEM unlocking is possible before purchase