Where are the email files located?

Nextcloud version (eg, 12.0.2): 13
Operating system and version (eg, Ubuntu 17.04): Ubuntu 17.X
Apache or nginx version (eg, Apache 2.4.25): Nginx, most up-to-date
PHP version (eg, 7.1): PHP, most up-to-date

The issue you are facing: Where are the files located for the emails that go out? I’d like to modify ours directly.

You mean you want to change the mail templates:
https://docs.nextcloud.com/server/13/admin_manual/configuration_server/email_configuration.html?highlight=mail%20template#using-email-templates