26 files declare their use of ContentEntityTypeInterface in Drupal 8

CommentStorageSchema.php in core/modules/comment/src/CommentStorageSchema.php

ConfigImportAllTest.php in core/modules/config/tests/src/Functional/ConfigImportAllTest.php

ContentEntity.php in core/modules/migrate_drupal/src/Plugin/migrate/source/ContentEntity.php

ContentEntityDeriver.php in core/modules/migrate_drupal/src/Plugin/migrate/source/ContentEntityDeriver.php

ContentLanguageSettingsForm.php in core/modules/language/src/Form/ContentLanguageSettingsForm.php

ContentModerationStateStorageSchema.php in core/modules/content_moderation/src/ContentModerationStateStorageSchema.php

content_translation.admin.inc in core/modules/content_translation/content_translation.admin.inc

The content translation administration forms.

DefaultTableMapping.php in core/lib/Drupal/Core/Entity/Sql/DefaultTableMapping.php

EntityConverterTest.php in core/tests/Drupal/Tests/Core/ParamConverter/EntityConverterTest.php

EntityFieldManagerTest.php in core/tests/Drupal/Tests/Core/Entity/EntityFieldManagerTest.php

Contains \Drupal\Tests\Core\Entity\EntityFieldManagerTest.

EntityTestUpdateStorageSchema.php in core/modules/system/tests/modules/entity_test_update/src/EntityTestUpdateStorageSchema.php

EntityTypeInfo.php in core/modules/content_moderation/src/EntityTypeInfo.php

entity_schema_test.module in core/modules/system/tests/modules/entity_schema_test/entity_schema_test.module

Test module for the entity API providing a bundle field.

FieldDefinitionIntegrityTest.php in core/modules/field/tests/src/Kernel/FieldDefinitionIntegrityTest.php

NodeStorageSchema.php in core/modules/node/src/NodeStorageSchema.php

PathAliasStorageSchema.php in core/modules/path_alias/src/PathAliasStorageSchema.php

RelationLinkManager.php in core/modules/hal/src/LinkManager/RelationLinkManager.php

ResourceTypeRepository.php in core/modules/jsonapi/src/ResourceType/ResourceTypeRepository.php

SqlContentEntityStorage.php in core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorage.php

SqlContentEntityStorageSchema.php in core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorageSchema.php

SqlContentEntityStorageSchemaTest.php in core/tests/Drupal/Tests/Core/Entity/Sql/SqlContentEntityStorageSchemaTest.php

system.install in core/modules/system/system.install

Install, update and uninstall functions for the system module.

system.module in core/modules/system/system.module

Configuration system that lets administrators modify the workings of the site.

TermStorageSchema.php in core/modules/taxonomy/src/TermStorageSchema.php

UserStorageSchema.php in core/modules/user/src/UserStorageSchema.php

View.php in core/modules/views/src/Entity/View.php