the_post_thumbnail('thumbnail'); add post id a href link php code example
Example 1: get featured image url
echo get_the_post_thumbnail_url();
Example 2: get featured image post wordpress by post id
<?php if (has_post_thumbnail( $post->ID ) ): ?>
<?php $image = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'single-post-thumbnail' ); ?>
<div id="custom-bg" style="background-image: url('<?php echo $image[0]; ?>')">
</div>
<?php endif; ?>