Query Set Issues

I’m using the pre_get_posts hook, and trying to define which categories are displayed on a specific page. I created a function in functions.php that checks to see if I’m on that page, but I can’t get the query to list the posts from an array of categories.

function games_category( $query ) {

	$categories = array ('random', 'games');

	if ($query->is_main_query() && is_category($categories)) {
		echo("This is being read"); //This was read in my test

		$args = array(
		     'category_name' => 'articles', //can't take array, want to use $categories, threw an error
			);

			//$query->set( 'args', $args ); //this did nothing
			$query->query_vars['category_name'] = 'random';  //can't take array, but setting a single category works
			return $query;
		}
	}
	add_action( 'pre_get_posts', 'games_category' );

Essentially, I don’t understand the query->set portion of this code. I was using ‘args’, $args at one point, but I don’t understand that, and I believe I found that on a forum. It currently did nothing, it simply displayed the default posts of the category, and not what I was specifying in $args.

I just want to set the query ‘category_name’ to an array of categories, but can’t figure out the syntax, or how to go about it best. The ‘args $args route would be better, as I could set other properties, but one step at a time.

Any ideas what I’m doing wrong?