Apple doesn't have a monopoly.
You're free not to buy an iPhone. Most people don't.
You're free not to buy an iPhone. Most people don't.
But that doesn't matter when it comes to antitrust laws[1]:
> Courts do not require a literal monopoly before applying rules for single firm conduct; that term is used as shorthand for a firm with significant and durable market power — that is, the long term ability to raise price or exclude competitors. That is how that term is used here: a "monopolist" is a firm with significant and durable market power.
[1] https://www.ftc.gov/tips-advice/competition-guidance/guide-a...