[capableweb]

The window manager is definitely a "default program" that is probably tightly integrated in various points of the OS. AFAIK, you can't even use the OS without having Finder running (not the actual window "Finder" but the process) as it's also involved in lots of things, like the desktop and such.

[ratww]

I believe Finder is not technically necessary, as seen on Apple's installers and restore mode (which are a smaller version of macOS). You can kill it with Force Quit, for example. It is indeed also the provider of the Desktop, but everything else works: Dock, Menubar, Command+Tab.

It is launched via /System/Library/LaunchAgents/com.apple.Finder.plist