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.

Google Fi isn't deprioritized. There are a few MVNOs that have the same priority as first party carriers.

Apple could definitely strongarm the carrier to make them highest priority.