Drupal core defines a plugin.manager.field.widget service. By default, this is handled through the \Drupal\Core\Field\FieldWidgetPluginManager class. This plugin manager defines the field widget plugins that should be in the Plugin/Field/FieldWidget namespace, and all the classes in this namespace will be loaded and assumed to be field widget plugins.

The manager's definition also sets \Drupal\Core\Field\FieldWidgetInterface as the expected interface that all the field widget plugins will implement. This is why most field types extend \Drupal\Core\Field\WidgetBase to meet these method requirements.

As field widgets are annotated plugins, the manager provides \Drupal\Core\Field\Annotation\FieldWidget as the class that fulfills the annotation definition.

The entity form display system uses the plugin.manager.field.widget service to load field definitions and add the field's element returned from the formElement method to the entity form.