I posted this elsewhere, but the thing that makes ipadOS not feel like a real OS, to me, is that it is designed like an appliance OS. It lacks a whole bunch of very specific general purpose OS features:
1. You can’t build software to run *on* the OS *in* the OS -- you have to use a tool suite on another OS.
2. You can’t install software onto the device except from one repository, and that repository has very strict rules about functionality that is permitted in packages it hosts. There are whole classes of software that are ”not present” on iPads not because ipadOS can’t run them, but because Apple’s App Store rules prevent them from being offered to iPad users to install. And the EU alternate app stores still allow Apple to arbitrarily control what software *those* stores offer (see the example of UTM, which uses a JIT)
3. You can’t modify most of the OS behavior via modules or plugins or extensions, and what you can modify requires explicit blessing and agreement from Apple. This is tied to #2, but is at a different layer. Think fonts or printer drivers or mouse button remapping. There are clumsy OS level abstractions for bits of this in ipadOS, but it’s more limited than in most general purpose OSes.
4. Interprocess communication and data access (ie, file system stuff) is very different than most general purpose OSes.
5. You have no arbitrary background processes — everything is killed at a moments notice when not in focus or when the device is slept. This blocks server processes — which is also part of the rules in #2 — and requires awkward workarounds in the few places it’s allowed to exist at all.
Additionally, you can’t run any non-Apple-blessed arbitrary OS on their hardware, even if you want to. They control when the device is obsolete, not you, based on when they stop supporting OS updates on it. Which a MacBook, you can install Linux or BSD or something on it when Apple stops supporting it. Not so with iPads. They effectively become e-waste en masse.
There are reasons for all these limitations — some security, some business, some “user experience” — but the fact they exist, and there is literally no way around them, makes ipadOS work very differently than a general purpose OS.
Very broadly, iPads work like games consoles — you need specialized hardware to write software for them, and you need to have a business relationship with Apple to sell that software, and their rules control what you the user or you the developer can do with the machines and the OS. Apple being Apple, many of their rules are far more restrictive or arbitrary than most of the games consoles (as I understand them).
All of that is Very Different from pretty much any general purpose OS.