You're missing the key point that Microsoft makes an OS designed to run on hardware they don't make. Apple makes the iP* hardware and the entwined OS to run on it, much like PlayStation, Xbox, etc. Apple isn't trying to elbow other people off someone else's hardware - but Microsoft did, and got slapped down for it.

Yes, somebody does "make an alternative". There are hundreds of smartphones which are, at base specs, little different from an iPhone and run a variety of viable competing operating systems.

A monopoly occurs when nobody else can compete via resources the monopoly company doesn't ever own, either because the monopoly buys out every option (usually at a loss) for the sole purpose of denying access, or by conspiring to otherwise block fair competition. MS was in violation because (to at least a significant degree) they made everyone buy Windows (or, in a somewhat misguided case, mediaplayer), even if the hardware manufacturer was deliberately not installing Windows on a machine which the customer did not want Windows on; MS was interfering in a transaction they had no right to. Apple, on the other hand, makes the hardware and OS, and has every right to not sell one without the other - and if you don't want one or the other, there are plenty of fairly competing alternatives.