MycomponentController class is missing the class prefix. It should be -
class MycomponentController extends BaseController
public function __construct($config = , MVCFactoryInterface $factory = null, $app ...
As far I am aware, feel free to correct me as I haven't played with Joomla 4 too much, the class naming conventions for controllers, models and views you need to use for Joomla to automatically resolve and load the related classes are the same as Joomla 3 and PSR/4 therefore your Controller and Model class name should be changed to class ...
Normally fields are added by the Fields plugin. You can trigger onContentPrepare event to run this and other content plugins:
JFactory::getApplication()->triggerEvent('onContentPrepare', ['com_content.article', &$article, &$article->params]);
Fields will be available via jcfields property on the article object.
Another option is to manually ...