Email templates are used to design the emails sent to users and admins. You can generally find them all under MEMBER MANAGER->SETTINGS->EMAIL.
The default contents for each email template is located under the plugins/simple-press-member-manager folder (and for components, under the plugins/simple-press-member-manager/add-ons folder.
However, when you start to make changes to the templates, your changes are stored in your theme folder – under themes/your-theme/simple-press-member-manager.
This has some ramifications. In particular:
Therefore, it is STRONGLY recommended that you use a child theme. Child themes inherit its parent design but when the parent is updated, changes in the child theme are not overwritten.
Now, you might be tempted to edit the email templates in the main plugin folder. If you do this you will also lose your updates when the plugin is updated. So it is best to use a child theme.