I wonder if you think other OSes are any different?
TempleOS is the only thing that comes to mind that doesn't fit your description and it's not practically useful.
Any sufficiently large codebase is a mix of ideas and concepts implemented by different people with different priorities over a large timespan and if you can fit the entire thing in your head it's not very interesting or complex.
Qubes OS, the Linux distribution aspiring to offer a reasonably secure operating system, pioneering a "every app runs in a virtual machine" approach in the Linux laptop/desktop space, tracks this at the following issue:
https://github.com/QubesOS/qubes-issues/issues/2890
QubesOS is Xen based. Not Linux.
The *BSDs, Mac, and Windows all keep critical code in the same tree as the OS.
Something like disk encryption would be immediately visible.
So you don't have this mess of 80 different distros with 60 different versions of systemd, 20 that don't use it, a million kernel versions and it's all thrown together in a Costco-sized trash bag and we call the output "Linux".
In my experience any software system (not just operating system) after crossing a certain limit on complexity and age looks exactly as hodgepodge of code pieces thrown together, sometimes from different sources even if developed by one org. All major OSs have long crossed those limits, I believe.
Windows for ages did not really keep all the code in one repo. There were like a dozen parallel repos for e.g. the shell, kernel, IE, etc. Also every feature was developed on team-level branches; integrating all those branches often caused unexpected bugs.