No device manufacturer would ever be permitted to own a national carrier like that, nor vice-versa. It gives too many opportunities for monopolizing the customer -- like there aren't enough of those already!
That depends on their negotiations, no? Fi has been fantastic for me in the years I've had it, far better than first party AT&T or Verizon for my use cases.
Presumably that's because Google was able to negotiate favorable terms from T-Mobile. I assume Apple would be even better at that.