Thank you for the quick respond!! . I actually continued trying and I got it working with filterBy by using the ‘*=’, method. Something like:
<?php foreach( $pages->find('films')->children()->filterBy('Actors',**'*=',**$page->id() ) as $co ): ?>
<a href="<?= $co->url() ?>"><?= $co->title()->html() ?></a>
<?php endforeach ?>
It seems to work fine. I will give also a try at your solution. It is a good way of understanding how everything with Kirby work. Thanks a lot!