18 files declare their use of BatchBuilder in Drupal 9

BatchBuilderTest.php in core/tests/Drupal/Tests/Core/Batch/BatchBuilderTest.php

BatchTestController.php in core/modules/system/tests/modules/batch_test/src/Controller/BatchTestController.php

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

Helper module for the Batch API tests.

ConfigSingleImportForm.php in core/modules/config/src/Form/ConfigSingleImportForm.php

ConfigSync.php in core/modules/config/src/Form/ConfigSync.php

DbUpdateController.php in core/modules/system/src/Controller/DbUpdateController.php

install.core.inc in core/includes/install.core.inc

API functions for installing Drupal.

locale.bulk.inc in core/modules/locale/locale.bulk.inc

Mass import-export and batch import functionality for Gettext .po files.

locale.compare.inc in core/modules/locale/locale.compare.inc

The API for comparing project translation status with available translation.

locale.fetch.inc in core/modules/locale/locale.fetch.inc

The API for download and import of translations from remote and local sources.

node.admin.inc in core/modules/node/node.admin.inc

Content administration and module settings user interface.

node.module in core/modules/node/node.module

The core module that allows content to be submitted to the site.

PrepareModulesEntityUninstallForm.php in core/modules/system/src/Form/PrepareModulesEntityUninstallForm.php

ReviewForm.php in core/modules/migrate_drupal_ui/src/Form/ReviewForm.php

update.authorize.inc in core/modules/update/update.authorize.inc

Callbacks and related functions invoked by authorize.php to update projects.

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

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

user.module in core/modules/user/user.module

Enables the user registration and login system.