Nothing, really. There's plenty that's restrictive about one of several app stores on the platform (the one offered by apple), but - that's a store.

Actually using the OS; you can do pretty much anything you want - up to and including installing practically everything ever written for linux (including entire window managers and the whole lot - I've seen full GNOME installs on top of MacOS). About the only thing you can't do is modify the proprietary binary blobs they give you, but that's just commercial software 101.

This dark idea of an "authoritarian apple" is the same sort of "conspiracy fantasy" that people project onto the motives of political parties they don't like. It's the same sort of "leaps of logic" to assume "oh, yeah, they restrict app store apps from adding kernel extensions, so — they must hate the idea of you having control over anything, so obviously you don't have root access to your own machine.". "Or okay, yeah, you still have that, but obviously they're about to rescind that, any decade now." They're not.

The prediction is wrong, because it's an extrapolation from a bad starting point — which is a total misread of their motives. The motive isn't about authoritarian control; it's about eliminating footguns.

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For example, it's awful nice to be able to have a power failure, have my machine boot back up, and see all of my windows from the prior session right where I left them, and even have all the data in them refreshed from an autosave. As a system API, not just a per-app thing. Or, it's nice to install a program, and know that doing so is totally self-contained; it's not barfing a bunch of (potentially incompatible) new library dependencies into /usr/ or whatever that could screw with something else I've got installed. That I don't have to sweat over it "altering something" in my system when I do the install process. There are a lot of things like this; things where I basically feel like I've got a "wingman" or someone watching my back, because the folks who wrote it were primarily concerned about designing it so I'm highly unlikely to screw myself (i.e. the polar opposite of `rm -rf`).

It's just way lower stress to be able to focus on the actual problem I'm working on instead of also having to second-guess if my machine's going to betray me. That peace of mind isn't just a fluffy emotional thing; it also reduces cognitive load so I can work more effectively.