"Abuse of a dominant position" is a concept from competition law. In Apple's home jurisdiction, the United States, this field is called antitrust and the concept of abuse of a dominant position is called (unlawful) monopolization.
Antitrust cases are legally complex, and there is usually significant room for debate about whether a particular company is breaking the law or pursuing its legitimate interests. There are a number of claims pending against Apple alleging unlawful monopolization of various iOS-related markets. Identifying the relevant market is another fundamental concept in competition law which can be hotly contested in litigation.
As of January 2023, the most authoritative legal opinion on the antitrust implications of Apple's control of iOS is the September 10, 2021 decision of the United States District Court for the Northern District of California in Epic Games v. Apple. That decision has been appealed to the Ninth Circuit Court of Appeals, which heard oral argument on November 15, 2022, but has not yet issued its decision.
Epic did not complain about the need to develop iOS apps on macOS. Rather, Epic says that Apple illegally monopolized its control over the "aftermarkets" for iOS app distribution and in-app payment processing. The district court rejected Epic's federal antitrust claim by adopting a definition of the relevant market which was closer to that advanced by Apple:
Ultimately, after evaluating the trial evidence, the Court finds that the relevant market here is digital mobile gaming transactions, not gaming generally and not Apple’s own internal operating systems related to the App Store. The mobile gaming market itself is a $100 billion industry. The size of this market explains Epic Games’ motive in bringing this action …
Having defined the relevant market as digital mobile gaming transactions … the Court cannot ultimately conclude that Apple is a monopolist under either federal or state antitrust laws. While the Court finds that Apple enjoys considerable market share of over 55% and extraordinarily high profit
margins, these factors alone do not show antitrust conduct. Success is not illegal.
The district court's decision was sharply criticised by the Department of Justice, the Electronic Frontier Foundation and Microsoft, who all filed amici briefs broadly in support of Epic's position. One commentator who has analysed the oral argument in the Ninth Circuit confidently predicts that "Apple will most likely lose this appeal" and "this case will presumably end up in the Supreme Court."
I will update this answer when a higher court determines the relevant market, under U.S. antitrust law, for assessing claims that Apple is unlawfully monopolizing its control over iOS.