it's the same as for a normal post - in postmeta, the meta_key _thumbnail_id - this gives you the post id of the attachment post
if you want the url, use wp_get_attachment_url( get_post_thumbnail_id() )
it's the same as for a normal post - in postmeta, the meta_key _thumbnail_id - this gives you the post id of the attachment post
if you want the url, use wp_get_attachment_url( get_post_thumbnail_id() )