使用Symfony Doctrine加入两个表

I'm learning symfony framework as of the moment and I am very new to this framework.

what I am trying to do is to join two tables named movies and genres

and what I want is to achieve this using the query builder and also what is the advantage of using the Query Builder?

as of the moment I am using this to query all movies:

$movies = $this->getDoctrine()
                ->getRepository('AppBundle:Movies')
                ->findAll();

I'm really new to this so please bear with me.