8 files declare their use of UpdateManagerInterface in Drupal 8

update.compare.inc in core/modules/update/update.compare.inc

Code required only when comparing available updates to existing data.

update.fetch.inc in core/modules/update/update.fetch.inc

Code required only when fetching information about available updates.

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.

UpdateContribTest.php in core/modules/update/tests/src/Functional/UpdateContribTest.php

UpdateController.php in core/modules/update/src/Controller/UpdateController.php

UpdateManagerUpdate.php in core/modules/update/src/Form/UpdateManagerUpdate.php