iPhones aren't a "market". If that were the case, then Amazon would have a monopoly on Amazon ad placements, and Microsoft would have a monopoly on Excel features.
In terms of overall downloads, Google Play Store probably exceeds what Apple's store processes.
Interestingly, lawyers/regulators can define a "market" any way they want. The courts can decide.
But, while iPhones are part of the "phone market", "iOS Apps" are their own unique market. Probably only a matter of time before regulators think of it that way.
In terms of overall downloads, Google Play Store probably exceeds what Apple's store processes.