help with tutorial: fallback thumbnail issue

im trying to figure out how i can manage to create a standard fallback image if a thumbnail is not given… or a thumbnail is yet to be set. (for an empty ad or something like that)

Currently using this improvised piece of code:

<?php if ( has_post_thumbnail()) : ?>

           <div class="caption_fading"><strong>&euro; <?php the_post_thumbnail_caption();?>,-</strong></div>
           <div class="post_image_fading"></div>
                     <?php the_post_thumbnail(array(150, 160, false)); ?>
            <div class="bg_post_image"></div>

         <?php endif; ?>

How would i integrate something like this into it.

https://www.wpbeginner.com/wp-themes/how-to-set-a-default-fallback-image-for-wordpress-post-thumbnails/

im kinda failing in my PHP attempts.. any advice given is welcome, thanks.