The relevant market for them is smartphones and smartphone apps, and again, Apple doesn't have a monopoly there. Most markets have concentration effects and players that dominate the market to a certain degree, that doesn't automatically make them a monopolist, it depends on how they got there and what exactly they do to keep or build out their position in the market. By your definition Valve is a monopolist in the gaming market due to their size and dominance, but that's likely not true either.

Anticompetitive behavior would be if they used their power to make it more difficult for people to buy Android phones e.g. by entering into exclusivity deals with cell service providers or electronics stores so that you could only find Apple products there (i.e. T-Mobile would only sell iPhones with their contracts and you wouldn't find any Android phones except in some small speciality shops out of town). That's what Google is doing in its markets among other things, i.e. pay tons of money to ensure all virtual store fronts are only stocked with Google products and everything else is hidden behind.

Apple does of course show anti-competitive behavior to a degree, i.e. they purchase the entire production capacity of the most advanced semiconductor fabs to have exclusivity and preserve their edge, but again there are still other players in that market and competition still seems possible. If you want to compare that to what Google is doing in the search and ad space it would translate to them locking up almost all semiconductor suppliers in exclusivity contracts for 10 years so that no other company could ever build any advanced chips in large numbers.