Apple stores only sells accessories to their manufactured products, and isn't a retail platform.

If Apple opened stores that sold a wide range a goods, including from other sellers, and then banned Dell, then yes, your hypothetical would be anticompetitive.

Then why aren’t apps an “accessory” to the iPhone where Apple can decide not to allow certain apps - including hypothetically Amazon? Do the console makers have a right to deny certain games on their platform?

Does a physical store have to sell every product imaginable?

When you buy an app from the App Store. Apple claims the entire price of the app as revenue, pays out the developer and gives the developer a 1099 at the end of the year. My relationship is no more with the third party app developer when I buy an app from the App Store than it is if I buy a third party accessory from the Apple Store or if I buy something from Walmart.

There is no difference.