Check out example codes for "get featured image post wordpress by post id". It will help you in understanding the concepts better.

Code Example 1

echo get_the_post_thumbnail_url();

Code Example 2

<?php 
  // ALL parameters are optional. Not needed if you are on WP LOOP
  echo get_the_post_thumbnail_url( $the_query->ID, array( 500, 400) ); ?>

Code Example 3

<?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; ?>

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