OS reverse engineering?

Question

Operating Systems

Hello. Several questions about closed operating systems are blowing my mind. What is the difference between decompiling an OS and decompiling a regular program? Why has no one been able to restore the source of IOS (even the oldest versions) or Windows so far? Please explain in as much detail as possible.
Thanks to all.
UPD1: Due to the fact that many do not know why the goal of decompiling the OS can be pursued - I explain: Search for holes, modification, etc. ...
UPD2: Thank you all for your replies. Theoretically, this is possible, but very costly. All clear.

Answer 1

if you have a lot of free time (a couple of three years) and you are well versed in hardware and the disassembly process, implement all the necessary tools to simplify the process, then write an iOS clone compatible with their software. Only you will either be sued as a result or nothing will come of you.

Answer 2

1. It's illegal
2. It's very expensive and labor intensive
3. It's very long

Answer 3

Decompilation is no different.
restoring the source code is a thankless and unnecessary task. Firstly, the compiler generates DLL instructions into assembler ones and it is possible to find matches for the necessary instructions. But while you are looking for these matches, you will no longer need to decode in Java - you will perfectly begin to understand everything in assembler.
Secondly, what is the goal to be pursued? Resale? This is illegal and punishable. And even without explanatory comments, this is of little interest.
Thirdly, it is desirable to decode the sources before the release of a new version of the operating system.

Answer 4

What for?
I'll explain. Why decompile os if its API and ABI are documented?

Answer 5

Why reverse, the iOS kernel is freely available :)
www.opensource.apple.com/source/xnu/xnu-1456.1.26
but Windows is actively reversed, see https://www.reactos.org/