Having bought many pcs and Macs, you're wrong. You can buy a computer from a box brand with or without the OS. If you purchase it with, then you pay the cost of the OS on top. It's a separate charge. You cannot purchase an Apple computer or device without an OS. You also do not purchase the OS or a license of it. Not since, I believe, OSX Tiger. Since whichever it was, you no longer purchase a license for macOS and you no longer own it in any fashion. There is no charge for it, hence the free downloads and upgrades. And again, you need to consult your contract and TOS if you don't believe me.
Just because you can buy Windows separately as well bundled with hardware while iOS is only sold bundled with the hardware does not mean that you aren't purchasing the software (license) in BOTH of these incidences. "Ownership" of the operating system doesn't automatically change because it was purchased separately versus bundled with the hardware. Also, the fact the Apple stopped charging for OS X upgrades after 2012 didn't necessarily mean users "owned" the operating system any more or less than before that time.
Again, Apple is indeed selling the OS as part of the purchase of an iPhone, iPad, Mac, etc. just as can be the case when buying a PC.
Which is? Monopoly, duopoly, etc have hard definitions. "Dominant" does not.
Monopoly and duopoly don’t necessarily have “hard” definitions which is a reason why there can be so much debate and legal wranglings about the terms. They can be defined different ways depending on the country/state/region, court/case, etc. Again, the EU's definition of "gatekeeper" is not any more nebulous than monopoly, duopoly, etc. definitions can be.
It quite literally is the deciding factor as auto manufacturers are the only ones who have MSRP laws. Why? Because they have dealers they contract with to sell their vehicles and they got in legal trouble for, as you put it, price fixing. They were marking up prices via dealerships and getting the kickback. So the MSRP came around to show what the cost of the vehicle off the line was plus what the dealership is marking it up to. Which still isn't what you pay but that's not the argument, but a gripe.
In a contracted retailer situation that is exactly the case. Apple does not dictate how much an app is sold for. Walmart does not dictate how much a bag of Fritos is sold for. They have a contract with the app dev or Frito Lay to take a percentage cut of sales or a monthly contracted amount for shelf space that does not affect the pricing of the product. Same goes with Amazon as a retailer. They do not and cannot set the price for your product. They can only take a percentage of sales. Having sold many items in person, on Amazon, via Apple App store in the form of books and apps, you are factually incorrect in stating that stores set pricing and can change it arbitrarily.
Any retail product can have an MSRP and the vertical price fixing laws can apply to many more products than just automobiles. Once again, retailers like Walmart absolutely can have control over the pricing of products they sell. Manufacturers of all kinds may set an MSRP (manufacturer's suggested retail price) but that doesn't necessarily mean retailers like Walmart have to stick to that price. Also, stores absolutely can work with sellers, developers, etc. regarding potential sales, discounts, promotions, etc. to entice people to use their store and/or make it more appealing to sell through their store.