Type A. Type B. Save. Type C. Save. How do you get back to state “A” using git?

You do not have all your states available.

Maybe through autosave? If your editor makes a commit every time you're inactive for longer than X seconds that should solve that, which I'm pretty sure is how most editor undo stacks work at some level.

I just tried it in Nextcloud Notes on Android (FlorisBoard has undo/redo buttons), and it seems to work this way. If I type a bunch of words quickly and hit undo, it removes all of them. If I stop for a couple seconds between each word, it undos them one-by-one.