Displaying Custom Post Meta

To modify the content of a post, you can use the_content filter. You (probably) only want to add the post meta to the main query. If not, you can skip the is_main_query() conditional. Then you can just get the post meta and format it how you see fit and append it to the content.

//* Add filter to the_content if we're in the main query
add_action( 'the_post', 'wpse_261935_the_post' );
function wpse_261935_the_post( $post ) {
  if( is_main_query() ) {
    add_filter( 'the_content', 'wpse_261935_the_content' );
  }
}
function wpse_261935_the_content( $content ) {
  //* Make sure to add and remove filter for each post
  //* to make sure it's in the main query
  remove_filter( 'the_content', 'wpse_261935_the_content' );

  $wpse_261935_meta = get_post_meta( get_post()->ID, '_wpse_261935_post_meta_key', true );
  $wpse_261935_content = wpse_261935_format_post_meta( $wpse_261935_meta );

  return $wpse_261935_content . $content;
}
function wpse_261935_format_post_meta( $post_meta ) {
  //* Format the post meta however you'd like
  return $html;
}