Spotify filed a complaint with the EU about Apple's practices and claimed Apple is stifling competition. There are a few claims in Spotify's case that lack any proof at the moment but there core issue with Apple seems to be pretty damning.
Apple takes 30% of every payment through its in-app payment system. When you're using someone else's service you should expect to pay a fee but Apple doesn't allow apps to use any other in-app payment system. Maybe you won't do it in-app. But they don't allow you to direct users to payments on websites either.
I saw many comments claiming Spotify simply doesn't want to pay Apple what they're owed. They can't make enough money so they're going after Apple.
But that misses the biggest problem with Apple's tactics. Even if you accept that the 30% fee is fair to operate within Apple's ecosystem (it's not), it's made even worse when Apple begins to compete with the services they force to pay their fee.
Apple Music doesn't have to pay 30% on every subscription every month. Competitors like Spotify are forced to either absorb the losses with their regular price or raise their prices and turn away customers. It's a clear abuse of Apple's power in the iOS ecosystem to gain market share in another.
In a fair system, only apps that use Apple's payment systems would pay that fee. App developers would be free to use their own payment systems or third parties like Stripe. Apple would still have advantages in markets like music streaming but it'd be a lot fairer.