When people say, they can contribute via email, does this mean they'll zip altered files and mail to the repo owner?

Git has a set of commands that can either provide ready to be sent email messages as files (if you have a proper client, not the web ones), or send the formatted email for you. Most good clients have a way to directly pipe messages to the shell, so you can execute git directly from there to apply the change. Quite streamlined once you've got everything setup (not that difficult) [0].

But it does requires people to be disciplined with their changes (no wip commits). This may require learning about the flags for `git rebase` and `git commit`.

[0]: https://git-send-email.io/

No. You'd send 'patches' via email. Each patch roughly corresponds to a commit. These are diffs + commit metadata that are directly formatted as email messages. A single submission of multiple commits will be made of multiple threaded email messages. The maintainer then 'applies' the received patches to their repo to recreate the commits from the contributor. I know that this sounds complicated, but it's quite streamlined in git. You don't have to do all that manually.

There is one more way to contribute by email. And they are... surprise! Pull requests! You can send pull requests to the maintainer via email, as long as your own modified clone repo is hosted and accessible online somewhere. It doesn't have to be on the same server (unlike github forks). This is done using the `git request-pull` command.

Wouldn't they use git format-patch?