You can buy whatever phone you want. Many of them can run multiple OSes if that’s what you want. There is not a single mobile platform in town.

When Ford sells me an F150 I don’t cry about them having a monopoly on their Sync platform. That’s not now monopolies work

That's not a good analogy for the situation. A better grocery-based analogy would be if there's only a single Bob's Grocery in town (and for whatever reason more Bob's Groceries can't be opened) but there is a Sally's Groceries in town. Then Bob's Groceries isn't a monopoly. The only thing Bob's Groceries is a monopoly of is Bob's Groceries, which is a useless tautological argument.

Apple has neither a monopoly on phones nor on app marketplaces.