The interface translation module provides \Drupal\locale\LocaleTranslation, which implements \Drupal\Core\StringTranslation\Translator\TranslatorInterface. This class is registered under the string_translation service as an available lookup method.

When the t function or the \Drupal\Core\StringTranslation\StringTranslationTrait::t method is invoked, the string_translation service is called to provide a translated string. The string_translation service will iterate through the available translators and return a translated string, if possible.

The translator provided in the interface translation will then attempt to resolve the provided string against known translations for the current language. If a translation has been saved, it will be returned.