Everyone has a monopoly on their own products. Samsung has a monopoly on Galaxy phones. Tesla has a monopoly on Model Ss. It’s meaningless to say that Apple has a monopoly on a certain part of the iPhone infrastructure, since it’s their product.
Of course they don’t have a monopoly on mobile apps or app stores or phones. There are competing products.
You're conflating the phones themselves with the secondary markets they create.
The problem is that Apple has a monopoly in selling/distributing Apps to iphone users. This is not the case on other operating systems, technically not even on android(eg. Samsung) and has nothing to do with a "monopoly on their own products".
Of course they don’t have a monopoly on mobile apps or app stores or phones. There are competing products.