If you have an iPhone the Appstore is unquestionably a monopoly. At least Google can make the case you can install other app stores so the monopoly claim is weaker.
Does Apple also have a monopoly on writing the OS that runs on iPhones?
Do they also have a monopoly on creating the iPhone hardware? And choosing what they write in press releases?
Yes, of course. But I think the issue might be the extent to which this is an artificial monopoly, rather than a natural one.
A perfectly suitable competitor ... which has exactly the same fees, similar policies and do not seem to have any pressure due to the competition to change them, yeah something does not sound right here.
Alternatively I could compare the number of companies I could use to host my web app (100k+) to the number of companies I could use to host my mobile app (just 2). The lack of competition when you compare that to an healthy market is obvious.