So from a regulatory standpoint, Apple is the problem child even if it isn't a monopoly. The EU sees a third of its phone-using population as being "captured" by a uncompetitive foreign corporation that is far more restrictive and locked down than any of its competitors. Apple has also tenaciously resisted any attempts to open its platform by citing user security as a reason for its draconian level of control over the iPhone platform, so it became necessary for the EU to resort to powerful big-guns legislation to act. Well the big guns are here, and I don't think Apple's "user security" defense is going to be aegis enough against them.
Its amazing how some people are SO obsessed with their favorite brand that they cant see outside its limits: Apple controls ~30%, Google controls ~30%, thats pretty much majority of the 'market'. And that 'market' is actual people, having to daily use these devices.
There being 3-4 companies implementing exactly similar business practices does not make an 'open market'. It makes uncoordinated, non-conspiratorial monopolies.
https://gs.statcounter.com/os-market-share/mobile/europe
Further, other phone companies start from nothing and become quite successful in market place, and some fail, but consider OnePlus - they decided to make a "Flagship Killer" and are still delivering high quality devices.
They have monopoly power and control over app developers' access to iPhone users.
If supermarkets offered a loyalty card program that made it physically impossible for you to shop at another supermarket, then this would be a monopoly/anti-trust issue too, even if no supermarket had a majority share of the market.
The usual response of Apple fans is "You can just buy a second phone", but asking app developers to give away free Android phones to their iPhone-using potential customers is not really a viable competitive strategy.
If switching from an iPhone to an Android device required the user to jump through literal hoops, on camera, Apple fans would probably complain about governments forbidding hoop-jumping clauses in EULAs.
However, there are anti-competitive policies of Apple which burden not just app developers and users but Apple themselves, and it's really hard to justify their existence other than "monopolies are very profitable".
https://www.reddit.com/r/swift/comments/3d8koa/apple_rejecte...
https://www.ftc.gov/advice-guidance/competition-guidance/gui...