Append custom field items to content from plugin

You can use the the_content filter to add code directly into the content.

add_filter( 'the_content', 'wpse_202010_modify_the_content' );

function wpse_202010_modify_the_content( $content ) {
  global $post;

  if ( in_array( $post->post_type, array( 'post', 'page' ) ) ) {
    $content="<p>New content here</p>" . $content;
  }

  return $content;
}