9 files declare their use of FieldUI in Zircon Profile 8

EntityDisplayFormBase.php in core/modules/field_ui/src/Form/EntityDisplayFormBase.php

Contains \Drupal\field_ui\Form\EntityDisplayFormBase.

EntityFormDisplayEditForm.php in core/modules/field_ui/src/Form/EntityFormDisplayEditForm.php

Contains \Drupal\field_ui\Form\EntityFormDisplayEditForm.

EntityViewDisplayEditForm.php in core/modules/field_ui/src/Form/EntityViewDisplayEditForm.php

Contains \Drupal\field_ui\Form\EntityViewDisplayEditForm.

FieldConfigDeleteForm.php in core/modules/field_ui/src/Form/FieldConfigDeleteForm.php

Contains \Drupal\field_ui\Form\FieldConfigDeleteForm.

FieldConfigEditForm.php in core/modules/field_ui/src/Form/FieldConfigEditForm.php

Contains \Drupal\field_ui\Form\FieldConfigEditForm.

FieldStorageAddForm.php in core/modules/field_ui/src/Form/FieldStorageAddForm.php

Contains \Drupal\field_ui\Form\FieldStorageAddForm.

FieldStorageConfigEditForm.php in core/modules/field_ui/src/Form/FieldStorageConfigEditForm.php

Contains \Drupal\field_ui\Form\FieldStorageConfigEditForm.

FieldUiTest.php in core/modules/field_ui/tests/src/Unit/FieldUiTest.php

Contains \Drupal\Tests\field_ui\Unit\FieldUiTest.

field_ui.module in core/modules/field_ui/field_ui.module

Allows administrators to attach custom fields to fieldable types.