Just like the initial iPhone needed one carrier to get things right, 5G infrastructure testing will be like that.
It is in Apple’s best interest if their users experience 5G as best as the current infrastructure allows (which isn’t great yet, because it will take a decade for a full rollout).
It makes sense to have the carriers bend over backwards to see who gets to be the main partner for Apple, because they wouldn’t be able to manage 4+ relationships simultaneously with something as complex as 5G/4G flipping (this has to happen on device, because 5G drains battery 10x faster than 4G, so the device has to use bursts of 5G. If it handles this incorrectly, millions of users will complain of horrible battery life and that would be a big black eye for Apple, something they don’t want to risk).