Where do OTA updates come from?

Where do OTA updates actually come from?

For example let's say you own a Manufacturer ABC cell phone, have it running on Service Provider DEF towers, but are using MVNO XYZ to provide service.

When you perform an OTA update, who is actually providing the bits (the actual ROM) and who is transmitting it to you? I guess that's really 2 questions.

 

What about an unlocked phone that was originally offered by the carrier, but was only used on the MVNO (never directly on the carrier itself)?