https://eur-lex.europa.eu/EN/legal-content/glossary/competit...
All countries with a working democracy would class Apple as a monopoly as it has significant market power, enough to distort prices.
The market is mobile devices.
Now suppose you have an iPhone and want to download an app, or want to distribute your app to customers many of which have an iPhone. What substitute for Apple's store is available to do that?
Apple can change their rules and it materially impacts the market.
If it helps, think if Windows apps had a 5% Microsoft tax and had to be approved by Apple.
When they were found in the US to have a monopoly, they had around 95% of personal computer operating system share. Apple isn’t even close.
For iPhone apps Apple is way behind with about 0.005% of the total 2.17 million iPhone apps.
I.e. the monopoly question is: if apple takes X% for each app in this market, and a competitor can't take less, then it's a monopoly.