That's not a monopoly nor is that how monopolies work. iPhone is their device and creation, they can lock it down as much as they choose. They could only allow apps they develop if they wanted.
That's debatable. Just because the Play Store exists doesn't mean the App Store isn't its own market
The app store is not a free market. If I set up a card trading store, I'm free to set any constraints I want. Maybe I want to only allow users to trade cards that were purchased at my shop. That's my right as a private enterprise. Maybe everyone will hate it and go to a different store, and that's the free market. If I somehow prevented any other card stores from existing, then we would have a monopoly