@bogpunk in the US carrier devices come with locked bootloaders at minimum, with only certain phone manufacturers supporting manual appeals for unlock keys. Most don't allow unlocking it ever. This is different from "unlocked" as a generic term meaning "you can use another sim card" and way more pervasive.
If you don't have a choice to avoid carrier phones, you can try referencing a list that compares phone manufacturers by bootloader unlocking policies. They are sporadically made and may not be up to date, and your carrier may not offer or even support phones from any "unlock-friendly" manufacturers anyways.
The wiki advice is well worth heeding, but devices supporting GrapheneOS are expensive even secondhand. If you have budgetary concerns, LineageOS can support a wider range of budget-friendly used devices (provided they are bootloader unlockable).