the_post_thumbnail('thumbnail'); add a href link to post id php code example
Example 1: get featured image url
echo get_the_post_thumbnail_url();
Example 2: wordpress get fiture image
if ( has_post_thumbnail()) {
$large_image_url = wp_get_attachment_image_src( get_post_thumbnail_id(), 'large');
echo '<a href="' . $large_image_url[0] . '">';
the_post_thumbnail('thumbnail');
echo '</a>';
}