System administrators of a traditional Linux distribution assemble their own OS out of their package and configuration choices. There isn't a well defined standard base OS. That's part of what makes it the traditional approach and is inherently incompatible with the privacy and security approach of AOSP or iOS in many ways.

Linux distributions use different implementations of init systems, shells, command-line tools and nearly everything else. Ubuntu uses glibc, systemd and Rust uutils coreutils. Alpine uses OpenRC, Musl and BusyBox as the defaults. Debian uses glibc, systemd and GNU coreutils as the defaults but supports other choices of init system. Each has their own variants of the projects they each package with different versions, patches compile-time configuration and default runtime configuration.

Using systemd, Bash, etc. on an OS Debian is a choice for the system administrator rather than the OS being defined that way. Even if people swap out major components for ones which aren't officially supported, it's not generally regarded as not using the distribution anymore. It's a far different approach than defining a standard base OS, developing that together as a whole with user installed packages and configuration changes are solely on top of that.

The higher up you go in the software stack, the more different things are across operating systems. The Debian installations across different machines are a vastly different OS with far different components and configuration. There are default sets of packages and configurations but not a standard base OS shared across each machine. Swapping out components and changing the configuration isn't making it not Debian and is pretty much required.

A huge portion of server Linux uses musl and BusyBox due to Alpine containers.

Ah, I see, you mean an exclusive standard set of packages and its singular corresponding configuration.