what's are these "@name" and "@site-name" token-things in email_example.module and how do they work?
Please refer to the official D8 Sanitizing Output Documentation.

From the documentation page

There are three available placeholders in Drupal 8:


@variable: When the placeholder replacement value is a string or a MarkupInterface object
%variable: When the placeholder replacement value is to be wrapped in em tags.
:variable: When the placeholder replacement value is a URL to be used in the "href" attribute


You can learn more about these placeholders in the FormattableMarkup::placeholderFormat() documentation.

EDIT FOR DRUPAL 7:

See this documentation https://www.drupal.org/docs/7/api/localization-api/dynamic-strings-with-placeholders

The variables/tokens available for use as replacements is dependent on the context of the surrounding code and whether or not the script is called via Apache or from the command line.