It quite literally is a lisp runtime with text editing stuff. There are many resources on how to implement lisp, but for the text editor part, this book “The craft of text editing” was very good http://www.finseth.com/craft/

Problem it's an OS without multithreading[1]. One of the biggest Emacs problems, IMHO.

[1] https://www.reddit.com/r/emacs/comments/ghq1yx/lets_get_real...

Well, it's not exactly a great idea but you can replace init with Emacs and it will boot [1]. You have basic utilities like a shell, file manager and with plugins you can run a music player...

[1] http://www.informatimago.com/linux/emacs-on-user-mode-linux....

> It’s almost an OS in a sense within a text editor.

That's actually not a terrible articulation of its design philosophy.