Apple has a monopoly on iPhone software, and while it's true that users who dislike the App Store can switch to Android, there is an enormous cost to switching phone ecosystems.
If you don't like Comcast, you could move to an area with a different ISP, and there's a certain point where you probably would move, if Comcast's service became egregious enough. But I don't usually see this argument when people refer to Comcast as a monopoly, because it's understood that the switching cost is unreasonably high.
Monopoly's are whether or not a product or service dominates a specific sector, industry or market. A company can't have a monopoly over its own products which by definition would cover most product/service companies.
Given iOS market share is about ~25% of the Mobile OS Market, it currently doesn't qualify as a Monopoly although it's currently under EU investigation & Epic lawsuit which may rule differently.
But isn't iOS software a specific market? iOS might be an Apple product, but "iOS software" is made primarily by third parties. That software is exclusively sold by Apple, but that's exactly what I find anti-competitive!
Separately, it's worth noting that iOS has ~60% market share in the United States[1]. It's lower globally, as you pointed out, but I'd argue domestic market-share should be what matters in US-based suits.
No it's just a software platform that's open to 3rd parties to develop for, just like Android, Windows, macOS, Linux, Xbox, PlayStation, Nintendo, Smart TV's, Speakers, Watches, embedded platforms, etc. you could go as far and say that Cloud Providers or even Game Engines are software platforms.
But a single platform doesn't define a market, the prominent market iOS is in would be smart mobile OS's of which Android is apart of. Its 60% US Market share could be a determining factor in the US although they do have a clear unobstructed competitor in Android - time will tell as rulings from current investigations & trials get handed down. Of course it's not illegal to have a monopoly, only if its dominant Monopoly power position is abused for anti-competitive purposes, typically for creating a monopoly in a different market, but there are a few other abuses.
If you don't like Comcast, you could move to an area with a different ISP, and there's a certain point where you probably would move, if Comcast's service became egregious enough. But I don't usually see this argument when people refer to Comcast as a monopoly, because it's understood that the switching cost is unreasonably high.