https://eur-lex.europa.eu/EN/legal-content/glossary/competit...
And on the iPhone, you MUST use the Apple App Store, not just as a means to distribute your app, but as your payment provider. You can’t collect payment details yourself, use Stripe, Paddle, anything. You MUST use Apple.
You simply cannot enter business without paying Apple a share. That’s a monopoly.
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?
More specifically your question is like asking what if you have a car that requires diesel, what substitute is available to let you use cheaper gasoline?
Yea defining the market is 95% of anti trust cases. But asking yourself 'if Microsoft did what Apple is doing' should be enough to show at some point, it is a market.
It's like asking what the available substitute is for diesel fuel from Shell, to which the answer is diesel fuel from Chevron, BP, Sunoco, etc.
So now what are the available substitutes for distributing apps to devices that require iOS apps and can't run Android apps?
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.