Your analogies are highly broken.
You can mix packaged food with whatever you want. You can choose to eat only the left half of your TV dinner. In short, once it's yours, you can do what you like.
You can take a song and remix it to your heart's content. That is, unless the song contains DRM which prevents you from extracting the raw audio data. This is something most people around here frown upon rather strongly.
Nothing prevents me from taking pre-made clothing and using it in ways the manufacturer didn't envision and doesn't desire. I can use store-bought pants to make rope or filter water or anything I wish.
As for putting any engine you like in a car, I don't know where that came from. Of course you can put any engine you like in a car, as long as you have the resources and the skills.
Nobody is saying Apple must explicitly support third-party stuff like this. They shouldn't go out of their way for it. All we want is for them to stop going out of their way to prevent it.
It's not just a matter of Apple not inviting third-party OSes and bootloaders. The problem is that Apple locks down the platform using digital signatures so that the only way to use anything but an Apple OS or bootloader is to find and exploit a bug in Apple's lockdown code. There is no similar situation in the world of packaged food, music (except for DRM, which is generally considered bad), clothing, or cars.