Adding something to the_content

You mostly got everything right. But the code you posted also gave an error caused by an ’ char in your code.

add_filter ('the_content', 'references'); 
function references( $content ) {    

    // only run the code on singular views of the post type "my_custom_post_type"
    if( is_singular('my_custom_post_type') ) { // insert your custom post type here

        // get current post ID
        $post_id = get_the_ID();

       // get your custom field here using what you need;
       // maybe use: get_post_meta( $post_id, 'custom_field_name');
       // you showed a shortcode, than maybe use:
       // do_shortcode('[acf field="custom_field_name"]');
       $custom_field = get_post_meta( $post_id, 'custom_field_name');

      $content.= '<h3>References</h3>';
      $content.= $custom_field;
      $content.= '<a href="https://***.com/sources">Read more about our sources</a>';    
    }    

    return $content; 
}

For example if you replace my_custom_post_type with product (the WooCommerce post-type) you will only see this content on single product pages.

You should put this code inside your functions.php or better, inside a plugin.