My work MacBook was pulled from an original Air from something like 2015, to a 2017 Pro and currently my 2019 Pro.
So I’ve got apps installed on my Mac that have been installed damn near 10 years ago.
Ditto my home 2015 Pro was later on migrated to a M1 Air. Hell, I’ve still some 32 Bit Steam games that still somehow run on my Air (least Steam tells me they’re 32 bit).
We could play this game ad-infinitum, each finding a level of supposed “openness” but the basic facts are that neither Windows, nor MacOS are truly open.
If you want open, then Linux is always going to be in the answer somewhere. Not MS Windows. And not Apple MacOS.
As such both Windows and MacOS are closed source.
As for “opening up the OS” both are pretty gosh darned flexible and extensible wrt other features.
However being based upon a BSD core, MacOS has had access to the Unix command line natively since forever. For Windows one used to have to rely on CgyWin before the virtualized WSL platform came to be.
Whilst MacOS has the somewhat opaque ~/Library for storing user settings and data, it pales into comparison to the massively Opaque Windows Registry.
I’ve had had very few issues fixing app install issues with my Mac - with Windows I’ve had more than one occasion where I’ve had to do a complete reinstall of the OS due to the Registry being totally hosed to the point I couldn’t reinstall apps again.
I think Windows is up there with the open source OSs (Linux, BSDs, etc) on regular PCs are at the same end of "run anything you want from wherever you want it", iOS devices are at the other extreme of "only run things approved by Apple", Android devices are pretty closer to iOS because they make you jump through hoops and potentially lose access to various functionalities to install certain things or gain root access. Modern macOS, as far as I understand, is somewhere in the middle: you have to jump through quite a few hoops to install certain kinds of software, and a few aren't permitted at all I think (unsigned kernel modules?).
You can run Windows almost on any hardware. So it is much more open in general.
You can equally run almost any imaginable software on both operating systems (if we ignore the performance), but you have extreme difficulties to run macOS on most hardware.
