The fact that (baring exploitable bugs) you can only run Apple approved software on an iOS device pretty much makes it "THEIR machine" anyway.

Didn't the EU have something to say about that lately?

You might be thinking about the DMA, which adds a lot of responsibilities to "gateway" providers and platforms like Apple and iOS.

However the EU ruled many years ago on this subject (https://www.infosecurity-magazine.com/news/apple-iphone-jail...). In effect, jailbreaking and modifying software is perfectly legal, as long as it's not for copyright infringement.

You can get a free Apple developer account and run any software you build on your own devices.

The way that works is you pay apple an annual fee so that they will sign your applications using their key. Their devices still check that the application is signed by apple. I think you could reasonably say you have no control over that device you paid for if this is what you have to do to run your software on it.

You only pay a fee if you want to distribute. It's free to build your own code and run it on your own phone.

As long as it remains signed (and thus approved) by Apple.

imagine if you needed a free ford account to be able to run any backseat you want in your new car

Imagine a world without bad car analogies...