get all products that have at least one taxonomy product_tag

$args = array(
    'post_type' => 'product',
    'posts_per_page' => -1,
    'tax_query' => array(
        array(
            'taxonomy' => 'product_tag',
            'field'    => 'term_id',
            'terms'    => '20',
        ),
    ),
);
$products = get_posts( $args );