> I don't see them as market participant
But they are. There are competing apps to Apple's own apps on the App Store. Apple Music is an obvious example, but there's also web browsers, email clients, calendar clients, cloud storage, etc. Through their "market regulation" and using privileged APIs, they can artificially hold competitors back from reaching or surpassing parity of said competing apps.
Their "regulation" has forced all web browsers to use Safari on the backend without the benefits of things like content blockers. Their "regulation" has forced all apps to not be able to cut off a trial if they cancel the subscription renewal while hypocritically doing just that for things like Apple Arcade and Apple TV if you cancel the subscription before the trial is over. Their "regulation" has blocked out any competition to Apple Arcade like xCloud or Stadia. Their "regulation" has, until iOS 14 (still in a limited fashion), kept any other app from being a default and if those stock apps are deleted, annoy users to the point of reinstalling them due to popups when tapping an email address, date/time, or street address.
These are all ways that they secure their spot above the competition on their own devices and hold progression back until they're ready to implement competitive features (read: probably when competition starts poaching users from them despite the disadvantages they impose on said competition).
A participant in the market, which they most certainly are, should not be regulating that market and also profiting off of that regulation. This is how a manufacturer of only 3 types of devices is the most valuable company in the world. I can't think of any other company that has an entire market, or at least the size of App Store market, completely enveloped under their reign.