They dictate what you run on the phone if it requires using their native SDK. Aside from the Flash event almost a decade ago, they haven't tried to restrict web content
I haven't had an Android phone in almost a decade using just iPhones. iOS's web browser game is actually the most frustrating part about Apple's walled garden for me.
You can compile your own browser and install it. People act as if Apple is sending the police if they install an App that doesn't meet Apple's guidelines. Limitations are about the distribution on the App Store.
> Does Tim Cook pray at nights to slow down other peoples code on iPhone?
No, but the security team tries their best to keep unsigned, dynamically generated code (such as what might come out of your browser's JIT JavaScript engine) from running.
To see the differences between iOS Safari/webkit and Chrome on Android:
https://caniuse.com/#compare=ios_saf+12.0-12.1,and_chr+71
I haven't had an Android phone in almost a decade using just iPhones. iOS's web browser game is actually the most frustrating part about Apple's walled garden for me.