You don't see how you can be a monopoly with nearly 2/3 market share and 2.5x market size of your next-largest competitor? You must not be very imaginative

You're conflating two different markets.

Apple has less than 100% market share for phones. Apps and phone accessories are not phones, they're separate products made by separate entities.

If Apple phones and Android phones were compatible then the apps would be addressing the same market. For example, phones from Samsung and phones from Xiaomi both run the same apps, so they're in the same market. However, phones from Apple and phones from Samsung do not run the same apps. They're different markets. And Apple has a monopoly on the former.

Apple has majority share in the US.