Linux

Hey! Great questions.

It seems like what you're asking about are more what I'd think of as components of an a Linux "system" or "install."

First off, it's definitely worth saying that there aren't a lot of rules that would apply to "all" Linux systems. Linux is huge in embedded systems, for instance, and it's not terribly uncommon to find embedded Linux systems with no shells, no DE/WM, and no package manager. (I'm not 100% sure a filesystem is technically necessary. If it is, you can probably get away with something that's... kinda sorta a filesystem. But I'll get to that.)

Also, it's very common to find "headless" systems without any graphical system whatsoever. Just text-mode. These are usually either servers that are intended to be interacted with over a network or embedded systems without screens. But there are a lot of them in the wild.

There's also Linux From Scratch. You can decide for yourself whether it qualifies as a "distribution", but it's a way of running Linux on (typically) a PC (including things like DE's) without a package manager.

All that I'd say are truly necessary for all Linux systems are 1) a bootloader, 2) a Linux kernel, 3) A PID 1 process which may or may not be an init system. (The "PID 1 process" is just the first process that is run by the Linux kernel after the kernel starts.)

The "bunch of default applications and daemons" feels like three or four different items to me:

• Systemd is an example of an "init system." There are several available. OpenRC, Runit, etc. It's main job is to manage/supervise the daemons. Ensure they're running when they're supposed to be. (I'll mention quickly here that Systemd has a lot more functionality built in than just for managing daemons and gets a bad rap for it. Network configuration, cron, dbus for communication between processes, etc. But it still probably qualifies as "an init system." Just not just an init system.)
• Daemons are programs that kindof run in the background and handle various things.
• Coreutils are probably something I'd list separately from user applications. Coreutils are mostly for interacting with low-ish level things. Formatting filesystems. Basic shell commands. Things like that.
• User applications are the programs that you run on demand and interact with. Terminal emulators, browsers compilers, things like that. (I'll admit the line between coreutils and user applications might be a little fuzzy.)

As for your question about graphical systems, X11 and Wayland work a little differently. X11 is a graphical system that technically can be run without a desktop environment or window manager, but it's pretty limited without one. The DE/WM runs as one or more separate processes communicating with X11 to add functionality like a taskbar, window decorations, the ability to have two or more separate windows and move them around and switch between them, etc. A Wayland "compositor" is generally the same process handling everything X11 would handle plus everything the DE/WM would handle. (Except for the Weston compositor that uses different "shells" for DE/WM kind of functionality.)

As far as things that might be missing from your list, I'll mention the initrd/initramfs. Typically, the way things are done, when the Linux kernel is first loaded by the bootloader, it an "initial ramdisk" is also loaded. Basically, it creates a filesystem that lives only in ram and populates it from an archive file called an "initramfs". ("initrd" is the older way to do the same thing.) Sometimes the initramfs is bundled into the same file as the kernel itself. But, that initial ramdisk provides an initial set of tools necessary to load the "main" root filesystem. The initramfs can also do some cool things like handling full disk encryption.

So, the whole list of typical components for a PC-installed Linux system to be interacted with directly as I'd personally construct it would be something like:

• Bootloader
• Linux Kernel
• Initramfs
• Filesystem(s)
• Shell(s)
• Init system
• Daemons
• Coreutils
• Graphical system (X11 or Wayland potentially with a DE/WM.)
• User applications
• Package manager

But techinically, you could have a functional, working "Linux system" with just:

• Bootloader
• Linux Kernel
• Either a nonvolatile filesystem or initrd/initramfs (and I'm not 100% sure this one is even strictly necessary)
• A PID 1 process

Hopefully this all helps and answers your questions! Never stop learning. :D