"If I buy a Tesla, I don’t expect to be able to install software from BMW, or use wheels from Maserati"
Your analogy isn't hitting the mark at all. Tesla doesn't have an app store (as of yet), iPhone has.
This is not about replacing the operating system in Apple products, but what is allowed in the app store.
All the web browsers on iOS are just skins on Apple's inferior WebKit browser engine. Originally - and probably still to some degree - Apple just wanted to deny browser-based games and such by committing to a subset of HTML5 features - to push users into App Store. Having fewer features results in fewer vulnerabilities at the cost of weaker usability but I, for one, would like better browsers than what Safari or Firefox Focus can offer in my iPhone.
Apple has some valid reasons for mandating use of their own frameworks.
Parents need parental control over kids' internet browsing and if Apple's parental control won't work with 3rd party browsers, well, there's a problem.
I think Apple is also requiring software developers to use the built-in crypto provided by iOS which is probably better than having every app having vulnerable openssl libraries. Then again... Google, Microsoft and other big players are at least as adept in providing security updates / features.