Apple is a monopoly, no one else makes Apple products but apple, Microsoft make PCs, but so do Dell, Lenovo and others, If Apple sold its OS separately, and it run on any PC hardware, i would agree, they are not a monopoly, but as it stands, they are using their position as the sole manufacturer of their components to lock out others, and if they move into a new area, like, oh lets say, headphones or fitness trackers, they suddenly exclude "competitors" from their own stores and either prevent, or make it hard to get their hardware or software to play well in the eco-system that they have created, and often stolen from .
Most companies are the sole manufacturers of their own components, so that's a fairly useless definition of monopoly.
Apple makes computers that run Windows, via Bootcamp, so in that regard, they are not a monopoly, but yet another PC manufacturer. Yes, you can choose to run OS X (or Windows or Linux, etc.) on your Mac, and nowhere else. Just like how most PC manufacturers have their own proprietary crap-ware they add on to Windows, that you won't get on another brand of PC.
And yes, if you have an OS X application, it will only run on OS X, and likely nowhere else, apart from any emulation environments I'm unaware of. But, one can also run a Java application equally on OS X as Windows or Linux or whatever other platform. And even Posix code, and many other types of programs will work on they same across platforms.
So, other things are not locked out from running on Macs, there's only some limited lock in of some Mac stuff not running elsewhere. Which is fair, because that would cost them money to port that over and maintain it and test it.
Again, with having their own stores, and not selling competitors' products, that's very typical. Sony stores sell Sony products, nothing else, just like thousands upon thousands of other stores. Sure, there are large chains like Walmart and Best Buy that sell many brands, but Apple isn't one of them, nor does it need to be. The Gap isn't some restrictive monopoly because it doesn't sell H&M clothing.
Apple doesn't make things hard to inter-operate, the reality is that it's intrinsically hard to make things inter-operate, so they just don't bother in many ways. They don't care to make 20 configurations kind of work together, like other vendors, they just try to make 2-3 configurations work together as seamlessly as possible. If you think 2-3 or 20 ways is easy, then you just don't understand technology at a fundamental level.
And where I've seen them actively block inter-operation, it was usually for necessary reasons. For example, contractual obligations for having access to media content, imposed by MPAA/RIAA, or stopping jailbrakes for security reasons. Even a lot of the blocking of other technologies in making iOS apps made sense later when they could quickly transition to 64 bit, other chip architectures, or go from Objective-C to Swift. Broadly supporting many configurations just would have kept them from being so nimble. Look at the decade or so transition from 32 bit to 64 bit in Windows, with no end in sight, as most apps are still 32 bit, iOS taking around 1 year to go from 32 to 32/64, and likely 64 only within the next 2 years. That couldn't have happened if half the apps were all in C++ using some random other toolkit that Apple doesn't control.
How hard is it for fitness trackers to inter-operate? Use Bluetooth and HealthKit...
