[lucideer]

You're using the term "OS" but you're not talking about OSes you're talking about kernels.

The OS is what comes with the computer or gets installed in the default setup if you're installing yourself. Most of it is userspace.

Distros like Arch & Gentoo allow you to build a custom OS which in doing so blurs the definition quite a lot, but ultimately when people say "macOS" that term includes quite a lot of userspace things. Bundled software is absolutely a core part of every popularly used definition of the word "OS".