GrapheneOS There's an "Add a number" UI in the Phone app within the Blocked numbers menu.
Actually I can't find the blocked numbers menu. The three vertical dots dropdown just has "Call history" and "settings", the options at the bottom are favorites, recents and contacts. None of these destinations appears to have anything either.
In your other reply you said this was the standard UI yet every regular Android device I've ever used has easy access to block the number, so I guess Android doesn't follow the referenced standard.