Yeah, the manufacturer provides the OS, carriers add crap, remove features, and delay updates(*), as well as locking the phone to their service.
(*) Though bizarrely non-carrier Samsung phones in the US get updates last, the complete opposite of other manufacturers and the Samsung in the rest of the world. I have heard that it's because they don't release updates for those phones until all of the main carriers have approved them, which they regard as less important than their own branded handsets, but can't prove this is true.