15 files declare their use of FieldConfigInterface in Drupal 9

comment.module in core/modules/comment/comment.module

Enables users to comment on published content.

config_translation.module in core/modules/config_translation/config_translation.module

Configuration Translation module.

field.module in core/modules/field/field.module

Attach custom data fields to Drupal entities.

FieldConfig.php in core/modules/field/src/Entity/FieldConfig.php

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

FieldConfigListBuilder.php in core/modules/field_ui/src/FieldConfigListBuilder.php

image.module in core/modules/image/image.module

Exposes global functionality for creating image styles.

ImageFormatterBase.php in core/modules/image/src/Plugin/Field/FieldFormatter/ImageFormatterBase.php

layout_builder.module in core/modules/layout_builder/layout_builder.module

Provides hook implementations for Layout Builder.

media.module in core/modules/media/media.module

Provides media items.

MediaUiJavascriptTest.php in core/modules/media/tests/src/FunctionalJavascript/MediaUiJavascriptTest.php

MigrateBlockContentBodyFieldTest.php in core/modules/block_content/tests/src/Kernel/Migrate/MigrateBlockContentBodyFieldTest.php

MigrateFieldInstanceTest.php in core/modules/field/tests/src/Kernel/Migrate/d7/MigrateFieldInstanceTest.php

MigrateNodeTypeTest.php in core/modules/node/tests/src/Kernel/Migrate/d7/MigrateNodeTypeTest.php

views.views.inc in core/modules/views/views.views.inc

Provide views data that isn't tied to any other module.