You'd run "git format-patch" to create a .patch file with the diff of changes for a commit, and then run "git send-email" to send an email with the patch as the contents (optionally with additional comments). The recipient can then run "git apply" to apply the patches to their tree and push the changes.

For "private" repos, the recipient would be a specific repo maintainer/owner. For "public" repos like the Linux kernel, the recipient is usually a publicly-archived mailing list.