19 files declare their use of FieldConfigInterface

CommentHooks.php in core/modules/comment/src/Hook/CommentHooks.php
ConfigTranslationHooks.php in core/modules/config_translation/src/Hook/ConfigTranslationHooks.php
EntityBundleInfoAlter.php in core/modules/media/tests/modules/media_test_oembed/src/Hook/EntityBundleInfoAlter.php
field.module in core/modules/field/field.module
FieldBlock.php in core/modules/layout_builder/src/Plugin/Block/FieldBlock.php
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
FieldHooks.php in core/modules/field/src/Hook/FieldHooks.php
FieldStorageConfigEditForm.php in core/modules/field_ui/src/Form/FieldStorageConfigEditForm.php
ImageFormatterBase.php in core/modules/image/src/Plugin/Field/FieldFormatter/ImageFormatterBase.php
ImageHooks.php in core/modules/image/src/Hook/ImageHooks.php
LayoutBuilderHooks.php in core/modules/layout_builder/src/Hook/LayoutBuilderHooks.php
MediaHooks.php in core/modules/media/src/Hook/MediaHooks.php
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
NumberFieldTest.php in core/modules/field/tests/src/Functional/Number/NumberFieldTest.php

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