Why are you using $wpdb? This is a simple query,
check the ACF docs for usage examples (https://www.advancedcustomfields.com/resources/query-posts-custom-fields/)
<?php
// args
$args = array(
'numberposts' => -1,
'post_type' => 'product'
);
// query
$the_query = new WP_Query($args);
?>
<?php if( $the_query->have_posts() ):?>
<ul>
<?php while( $the_query->have_posts() ) : $the_query->the_post();?>
<li>
<?php the_title();?> - <a href="https://wordpress.stackexchange.com/questions/285872/<?php the_field("product_url');?>"><?php the_field('product_url');?></a>
</li>
<?php endwhile;?>
</ul>
<?php endif;?>
<?php wp_reset_query();?>