28 files declare their use of SelectInterface

AliasRepository.php in core/modules/path_alias/src/AliasRepository.php
block_content.module in core/modules/block_content/block_content.module
Allows the creation of custom blocks through the user interface.
Cache.php in core/lib/Drupal/Core/Cache/Cache.php
CachePluginBase.php in core/modules/views/src/Plugin/views/cache/CachePluginBase.php
CommentSelection.php in core/modules/comment/src/Plugin/EntityReferenceSelection/CommentSelection.php
Condition.php in core/lib/Drupal/Core/Entity/Query/Sql/pgsql/Condition.php
Condition.php in core/lib/Drupal/Core/Entity/Query/Sql/Condition.php
ConditionAggregate.php in core/lib/Drupal/Core/Entity/Query/Sql/ConditionAggregate.php
database.api.php in core/lib/Drupal/Core/Database/database.api.php
Hooks related to the Database system and the Schema API.
FieldOrLanguageJoin.php in core/modules/views/src/Plugin/views/join/FieldOrLanguageJoin.php
JoinPluginBase.php in core/modules/views/src/Plugin/views/join/JoinPluginBase.php
MenuTreeStorage.php in core/lib/Drupal/Core/Menu/MenuTreeStorage.php
node.module in core/modules/node/node.module
The core module that allows content to be submitted to the site.
Node.php in core/modules/node/src/Plugin/migrate/source/d7/Node.php
Node.php in core/modules/node/src/Plugin/migrate/source/d6/Node.php
NodeComplete.php in core/modules/node/src/Plugin/migrate/source/d7/NodeComplete.php
NodeGrantDatabaseStorage.php in core/modules/node/src/NodeGrantDatabaseStorage.php
NodeRevision.php in core/modules/node/src/Plugin/migrate/source/d6/NodeRevision.php
Query.php in core/lib/Drupal/Core/Entity/Query/Sql/Query.php
SearchQuery.php in core/modules/search/src/SearchQuery.php
SelectionInterface.php in core/lib/Drupal/Core/Entity/EntityReferenceSelection/SelectionInterface.php
SelectionPluginBase.php in core/lib/Drupal/Core/Entity/EntityReferenceSelection/SelectionPluginBase.php
SqlBaseTest.php in core/modules/migrate/tests/src/Kernel/SqlBaseTest.php
Contains \Drupal\Tests\migrate\Kernel\SqlBaseTest.
Tables.php in core/lib/Drupal/Core/Entity/Query/Sql/Tables.php
Tables.php in core/modules/workspaces/src/EntityQuery/Tables.php
Update.php in core/modules/pgsql/src/Driver/Database/pgsql/Update.php
UserSelection.php in core/modules/user/src/Plugin/EntityReferenceSelection/UserSelection.php
views_test_query_access.module in core/modules/views/tests/modules/views_test_query_access/views_test_query_access.module
Module to test entity query access in Views.

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