There are numerous companies that make bleach or wheat or ceramic bowls. Their products are completely fungible with their competitors. None of them has a monopoly on any of that.
Whereas the ones that have no viable substitutes for a particular use... have a monopoly. That's what a monopoly is. It's when you can't get something from anyone else.
That's not the same thing as having a vendor-specific part. It's pretty common that the brake pads for a particular vehicle won't fit on a car from any other make. However, that doesn't mean the OEM is the only one who makes that part. You can get Ford-specific brake pads from Duralast or Powerstop or others in addition to getting them from Ford.
> Ford would never sell you a car with an engine in it which isn't from them.
Ford has sold millions of cars with engines in them from Mazda and others.
> Apple doesn't prohibit you either from ripping out the motherboard from the chassis of whatever device and putting your own stuff in there. People change batteries etc.
They expressly prohibit you from running macOS or iOS on anyone else's hardware, and likewise prevent you from running any other OS on an iPhone.