Prison Garden
Apple Phone's App Store is a Walled Garden.
A Garden is nice place to be, but requires constant maintenance (by Apple) to keep it free of weeds. This is a good thing - the opposite is a wild thicket.
It is Walled, because you cannot escape. Visitors must stay within the walls. They cannot go elsewhere. This is bad.
So, can we have the best of both worlds? Yes of course.
I use Debian (a flavour of Linux). It has an "App Store", but it a called a "Software Repository". It is a garden, because the kind, clever and dedicated Debian folks carefully maintain the repository.
It is a pleasant place to be, with beautiful software, and very few weeds.
It is unwalled, because I am free to go elsewhere, I am not forced to use only software curated by Debian. I can add extra repositories. If Oracle, Apple, Google or any other organisation created their own repository, I would then see all of the software curated by many gardeners. The barrier to entry is low - anybody can set up a repository.
Walled Prison
IMHO, the term Walled Garden doesn't quite describe Apple's App store. It is a Walled Prison. Not only are you walled in, but there is also strict controls on what is allowed in the garden. They control every aspect of the garden with paid guards.
If it were a Garden (a nice place to be), then I would be able to choose the best software and content. I can't. Stretching the prison analogy, only cigarettes and stamps are allowed past the guards!
FYI, in this analogy, you (the user of an Apple Phone) are a prisoner, and I (a software developer) am a visitor, trying to improve the lives of the inmates by bringing luxuries into the prison.
Does it Feel Like a Prison?
Most people don't feel like they are in a prison. This is for two reasons.
1) They haven't hit one of the restrictions yet.
2) They haven't experienced freedom, and don't know what they are missing.
My Freedom and my Prison
I've been using Linux for decades now. I'm so pleased to have escaped Microsoft's prison (ironically called Windows). I own my computer. I can do with it what I want, and there's no prison guards enforcing their will on me. The thought of returning to Windows is dreadful.
Alas, my phone is an Android phone, and while the walls aren't a high, and inescapable as Apple's, there are still walls. So I don't like my phone. I don't own it. Google controls many aspects of it. I use F-Droid (a rival to Google's Play Store), so I do have some freedom.
Here are a few of the walls :
I can't choose to receive updates of the base system from whoever I want. Therefore my phone will become obsolete before the hardware fails. (Neither google, nor my phone manufacturer have my best interests at heart, they have no incentive to keep the software up to date).
I don't control the core operating system. F-Droid only lets me add applications from 3rd parties. I cannot replace any care features of my phone from 3rd parties.
Google makes it hard (or impossible?) for me to make full use of my phone, without them spying on me. (I'm thinking about the location service here).
I can't audit the software on my phone. I don't have all the source code, and even if I did, I couldn't make changes, recompile, and then update my phone. I'm stuck with whatever I'm given.
I am forced to trust Google, I'm forced to trust my phone manufacturer. They are in control of my phone, I am not. I hope they don't abuse that power, but when they do, there is nothing I can do about it. FYI, Trust is an interesting word, which many people don't understand! See Trust. I Trust them, but I know their interests aren't aligned with mine, and therefore I know they are likely to act against my wishes.