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

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

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

When the user interface defines the available fields, the plugin.manager.field.field_type service is invoked to retrieve a list of available field types.