Although the US antitrust laws do use the language of monopoly, they are broadly drafted and have been subject to a lot of judicial interpretation. Rather than monopoly, when considering antitrust questions it is sometimes better to think about "market power and … whether business conduct has or likely will have anticompetitive effects." [1]

"Market definition is least useful when market shares would not be strongly probative of market power or anticompetitive effect, while direct evidence as to market power or anticompetitive effect is available and convincing." Apple's control of the App Store has a direct anticompetitive effect in the secondary market for iOS apps. This is pretty clear from the fact that Apple explicitly prohibits some apps that compete with its own. The interesting question is whether the plaintiffs will succeed in proving that Apple's conduct is illegal, under the prevailing interpretation of the US antitrust laws.

[1]: Market definition: an analytical overview (2007). https://digitalcommons.wcl.american.edu/cgi/viewcontent.cgi?...

Why is "Apple devices" not a separate market when it comes to apps? Apps made for Apple devices can't be used on other devices without changes.

Many apps are available exactly the same on Android as well, and nearly all apps have at least some equivalent on Android. The fact that an iOS app isn't literally compatible with Android, you can't take the exact same binary and use it on an Android phone, doesn't mean that iOS is a separate market category. You may as well say that UPPAbaby strollers are their own market category because UPPAbaby stroller accessories don't work on other strollers.

> You may as well say that UPPAbaby strollers are their own market category because UPPAbaby stroller accessories don't work on other strollers.

I wouldn't say UPPAbaby strollers are their own market, but I would say UPPAbaby stroller accessories are their own market, at least if they have and act on a legal way to prevent all others from making and selling accessories that interact with their strollers.

It's the app market for iPhone users.

And notably Apple locks out some companies from the same kind of integrations that 1P apps get. The only use I have for Safari is that it’s the browser most apps open. And Apple won’t release Siri actions for music, putting Spotify at a big disadvantage.

"iPhone users" isn't a general market category.