The Synonyms are defined in the %PLUGIN_DIR%/Resources/config/Synonyms.php. The path to this file is saved in the swag_ses_synonym_dir parameter of the container and can be overridden with the default Dependency Injection. See How to override for more information.
The syntax in the association may look a bit strange, it's the Solr syntax.
The path parameter is afterwards passed to the Swag\EnterpriseSearch\Relevance\SynonymProvider class.