Link FROM attachment to full post and get custom fields values on attachment page?

If the attachment is attached to a post, the post is determined as “post_parent”.

So, if you are in the attachement template (attachment.php, image.php, etc):

while ( have_posts() ) {
  the_post();
  // Get parent id of current post (get_the_ID() gets id of current post,
  // whicdh is the attachement, as we are in the attachment template)
  $parent_id = wp_get_post_parent_id( get_the_ID() );

}

You can get the parent id also with this method, but I just prefer the other:

while ( have_posts() ) {
  the_post();
  global $post;
  $parent_id = $post->post_parent;
}

Once you have the parent post ID, you can get the link, custom fields or whatever you want of the parent post:

  if( $parent_id ) {
      // parent post has been found, get its permalink
      $parent_url = get_permalink( $parent_id );

       ?>
       <a href="https://wordpress.stackexchange.com/questions/217675/<?php echo $parent_url; ?>">
           <?php _e('Go to parent post', 'textdomain'); ?>
       <a/>
       <?php
       echo get_post_meta( $parent_id, 'custom-field', true );
  }