Parse args

Merge an array into a predefined one. Convenient to handle default configurations.

wp_parse_args( string|array|object $args, array $defaults = array() ): array

$args = wp_parse_args(

	array(
		'relation' => 'AND',
		array(
			'taxonomy' => 'product_type',
			'field'    => 'slug',
			'terms'    => 'variable',
			'operator' => 'IN'
		),
	),

	$query->get( 'tax_query' )

);

$query->set( 'tax_query',  $args );

Source: https://developer.wordpress.org/reference/functions/wp_parse_args/