That's different from controlling what YOU as a user can install on your phone. Apple controlls what you can make people install on their phones.
These are vastly different things. You own your phone and you can do whatever you want with it. It's just that you can't use Apple's distribution channels to spread you code.
You also can't put your own code on it without a developer key, and Apple is the gatekeeper for getting a developer key.
Theoretically you could bypass everything by replacing the OS, except that you can't replace the OS without yet another key that Apple isn't sharing with anyone.
No, you don't have to hack anything, you can write your code and run it on your iPhone that's running the legit iOS without going through Apple's review process.
People who develop apps do it everyday many times because they need to run their code on devices. You don't ask apple for permission every time you press the build button.
You don't have to go through the review process for getting an app approved for the app store. But you do still have to apply for a developer key, and buy the necessary additional equipment (which can only legally be Apple hardware), and comply with the associated contractual agreement.
That does mean Apple has a nonzero amount of control over that channel, too.
Literally every piece of electronics in front of me right now is either an Apple device, or connected to an Apple device.
I'm not hating on them. I'm just advocating a realistic perspective on the situation. iOS is a walled garden. That some people (myself included) see that as a net positive doesn't make it not one.
These are vastly different things. You own your phone and you can do whatever you want with it. It's just that you can't use Apple's distribution channels to spread you code.