Honestly, what you're describing sounds nothing at all like what I've witnessed, but I can't claim to know the process used by every carrier in the world, so I'll have to admit there is a chance some carrier somewhere in the world is doing something very out of the ordinary.
What you're describing is typical of Android however. A process that allows a carrier to modify an OS image was most definitely never, anywhere in the world, ever applied to an iOS device, and I've never heard of it being applied to WP either. Carrier specific WP ROMS are assembled by the OEM, not the carrier. Carrier specific apps are typically not directly baked into the WP ROM either. Instead, carriers with their own WP apps just upload them to the store. OEMs bundle carrier specific configuration files together with their carrier specific firmware, in which they reference those apps, which are then downloaded during initial setup. That's the last and longest step of the initial setup.
I don't know, but what you're describing as a WP update process sounds totally foreign to me. That's really all I can say about it.