Baseband firmware and OS activation have nothing to do with the carrier, just like it didn't on RIM devices back in the day (which is probably the only somewhat comparable version of this).

Perhaps you are thinking about subscription activation (be it GSM or CDMA) and parameters for cell networks (which can indeed be consumed by the baseband, which will be running firmware supplied by the manufacturer, sometimes re-packaged in system images as done in OEM feature phones and many android phones).

Either way, macOS devices do the same thing (activation) as do iPads without cell networking. Same goes for radio firmware loading and updates. You'll find most wintel laptops doing the same for things like WiFi (regardless of softmac/halfmac/hardmac chips).