Even if you buy the phone from a third party, there are a bunch of custom configs for every carrier in the world in the android source code.
They're mostly mundane things like 'should we believe the language specified in the sim card' or 'what max MTU should be used on this network', but there are also more sensitive things like 'what types of tethering to allow' or 'should we allow the user to manually select which network to connect to while roaming.
They're mostly mundane things like 'should we believe the language specified in the sim card' or 'what max MTU should be used on this network', but there are also more sensitive things like 'what types of tethering to allow' or 'should we allow the user to manually select which network to connect to while roaming.