Symfony 4：在控制器和自动装配之外挣扎

I have done this countless times now and perhaps it's because autowiring and dependency injection is too complex for me and I just need to work around it, but after consistently searching, I'm still having problems understanding the right way to fix this.

I have this class where I'm trying to create a query from - this is all great and fine as long as I extend "Controller" - as soon as I stray from this, I either get:

$em = new EntityManagerInterface();
    $query = $this->em->createQuery('SELECT m FROM App:Main m')
            ->getResult(\Doctrine\ORM\Query::HYDRATE_ARRAY);
    print_r($query);

This is obvious, you cannot instansiate an interface

Cannot instatiate interface Doctrine\ORM\EntityManagerInterface

Or:

Use App\Util\Db\Dbhelper;
$db = $this->container->get(DbHelper::class);
$result = $db->allMains();

Too few arguments to function App\Util\Db|DbHelper::__construct(), 0 passed 1 expected

The other classes are expecting the dependency injection of EntityManagerInterface obviously. All help is greatly appreciated, I don't know why I cannot seem to wrap my head around this properly.

DbHelper Class:

namespace App\Util\Db;

use Doctrine\ORM\EntityManagerInterface;
use Symfony\Component\HttpKernel\Exception\NotFoundHttpException;
use App\Entity\Main;

class DbHelper
{
    protected $em;

    public function __construct(EntityManagerInterface $em)
    {
        $this->em = $em;
    }

    public function getNames()
    {
        $names = $this->em->getRepository(Main::class)->findAll();
        foreach ($names as $name) {
            echo $name->getName();
        }
    }
}