How to stop WordPress from breaking code

Have you seen these?
https://www.ads-software.com/plugins/simple-embed-code/
https://www.ads-software.com/plugins/insert-php-code-snippet/
https://www.ads-software.com/plugins/insert-code/

To make your own shortcode, set up a child theme and put something like this in functions.php:

add_shortcode('my_code_snippet', 'my_code_snippet');
function my_code_snippet() {
  $html = '<h3>blah blah</h3>';
  $html .= '<p>blah blah</p>';
  $html .= '<p>etc etc</p>';
  return $html;
}

For JavaScripts, make a plain text file with your JavaScript and upload it to the server. Then load the script into your page with this in functions.php:

add_action( 'wp_enqueue_scripts', 'add_my_script' );
function add_my_script() {
  $my_page_id = 57; // set to the required page id
  if ($my_page_id == get_the_id() ) {
    wp_enqueue_script( 'my-js', '/wp-content/uploads/my_script.js', array(), "1", true);
  }
}