by writing your diffs with red and black ink, like the Aztecs did

Awkwardly. Pretty much generate a patch and email it as an attachment.

ELI5: Got has tools built-in to put your changes into emails and send them. People can either bring them in or see the diff in the emails.

Reading and interactive tutorials:

1. https://useplaintext.email/
2. https://git-send-email.io/

Found a blog post that gives a quick overview of how to do git via email in general: https://peter.eisentraut.org/blog/2023/05/09/how-to-submit-a-patch-by-email-2023-edition

So at least from my understanding you’d make your changes, email the contents of the patch to the maintainer, and then they’d apply it on their side, do code review, email you comments, etc. until it was in an acceptable state.

There’s also the full kernel development wiki that goes into all the specifics: https://www.kernel.org/doc/html/v4.16/process/howto.html

(I never got through the whole thing)