6 files declare their use of UpdateFetcherInterface

update.api.php in core/modules/update/update.api.php
Hooks provided by the Update Manager module.
update.compare.inc in core/modules/update/update.compare.inc
Code required only when comparing available updates to existing data.
update.install in core/modules/update/update.install
Install, update, and uninstall functions for the Update Manager module.
update.module in core/modules/update/update.module
Handles updates of Drupal core and contributed projects.
update.report.inc in core/modules/update/update.report.inc
Code required only when rendering the available updates report.
UpdateManagerUpdate.php in core/modules/update/src/Form/UpdateManagerUpdate.php

Buggy or inaccurate documentation? Please file an issue. Need support? Need help programming? Connect with the Drupal community.