This has been generally illegal in the US since 1975: https://www.law.cornell.edu/uscode/text/15/2302 (section c)
No, not really. In the EU for example, a unauthorized change to one part can legally never void the warranty to an unrelated part.
Here's IMHO a better analogy: Apple is running a mall, while also operating a chain store operating in said mall. Apple Mall allows competitors to Apple Chain Store to rent stores in the mall.
However, it turns out that for some reason, there are "Beware of Leopard" signs near the competitor stores. Also, the public area near them is never cleaned or maintained, except for signs promising cheaper products at the Apple Chain Store (and the leopard signs of course).
Would that be legal? Apple Mall is hardly a monopoly. Despite losing the location, the other stores could move. None of the agreements specifically bans this.
For some reason though, I don't think that would fly - the other stores have a reasonable expectation that after paying rent Apple won't sabotage them. Just like companies paying for the Developer License and Apple Store tax have a reasonable expectation Apple won't unduly hinder them for its own products.
This happens all the time. Integration of the radio and climate control system/navigation system. Custom wiring for the speaker system and accessory systems. Dashboard cavities which are a non-standard size.
Ultimately, being a marketmaker comes with responsibilities. If Apple wants to close iPhones and support only Apple software they could do that. But as long as Apple operates an App Store and does not allow other methods of install, this comes with the legal obligation to play fair.
Don't know where you live, but that is not true in the U. S. "It is generally accepted as" against Federal law.
Companies can say things they know aren’t true, misleading customers. They even often can make it hard for customers who know their rights to exercise those rights.
I think it would be an improvement if such actions were punished harsher and more often.
You have two system to choose from, Apple and Google.