People say "Arch makes you learn how Linux really works", but I don't think that's a good way to put it.

Arch teaches you about the most boring parts of how linux works: the specific idiosyncrasies and configuration minutiae of a ton of libraries and programs (GNU coreutils, systemd, udev, mesa, glib, X11, Xlib, xdg, dbus, etc.)

Arch will usually not teach you the really interesting "low level" things about Linux: How the kernel works, how threads are created, etc.