The text editor I'd used for 20 years on an MS-DOS platform would change the shape of the cursor to distinguish between insert mode and overwrite mode. A filled rectangle (or box, in Emacs terms) indicated overwrite mode; an underscore (hbar), insert mode. It's a nice way to make overwrite mode obvious.
The video starts out with Emacs in Insert Mode. Then Overwrite Mode is activated, which changes cursor-type from hbar to box. The word "Overwrite" replaces "Insert" to demonstrate the effect.
Incidentally, you might enjoy listening to some sound while playing back the video. One of my favorites is "Forest Voice" from https://mynoise.net, by Dr. Stéphane Pigeon.
Anyway, here's a function to implement that behavior in Emacs:
(defun zeit-change-cursor-shape () "Set cursor shape depending on overwrite mode. When overwrite-mode is nil (off), set cursor-type to hbar. Otherwise, set it to box." (interactive) (if (not overwrite-mode) (setq cursor-type 'hbar) (setq cursor-type 'box)))