Failed to instantiate component or class

THE EROR SHOWS AS WHILE TESTING DEPENDENCY INJECTION

Failed to instantiate component or class “\app\components\First”.

Caused by: ReflectionException

Class \app\components\First does not exist

in D:\xampp\htdocs\yii\newbasic\vendor\yiisoft\yii2\di\Container.php at line 447

WITH RESPECT TO ROOT , where is the location for component folder .? Are they under vendor subfolder ?