Send custom WooCommerce mail with user credentials after order with certain product

Hey for the question "how I can add my custom email-message with the credentials and a custom login link",
In your code, there is this line

$template = 'emails/my-custom-email-i-want-to-send.php';

you could add the credentials there, you may pass other arguments as the second parameter of this functionwc_get_template_html
second, the code order will not work for what you are trying to achieve

in this line, you are sending the email

$mailer->send( $recipient, $subject, $content, $headers );

And after that, there is a foreach loop for all the items in the order and creates a function looking if the user exists.

foreach ( $items as $item_id => $item ) {
       $product_id = $item->get_variation_id() ? $item->get_variation_id() : $item->get_product_id();
       if ( $product_id === XYZ ) {
            //get the user email from the order and check if registered already
            function email_exists( $order_email ) {

This function is not been called.

I will suggest to take out that function from the foreach loop and call it inside if your goal is to check only if the product XYZ exists

and all of it before the email

    function check_order_product_id( $order_id ){
  $order = wc_get_order( $order_id );
  $items = $order->get_items();
  $order_email = $order->billing_email;

  //create custom mail
  function get_custom_email_html( $order, $heading = false, $mailer ) {
    $template = 'emails/my-custom-email-i-want-to-send.php';
    return wc_get_template_html( $template, array(
        'order'         => $order,
        'email_heading' => $heading,
        'sent_to_admin' => false,
        'plain_text'    => false,
        'email'         => $mailer
    ) );
  }
  function send_custom_email($order_email, $order){
    // load the mailer class
    $mailer = WC()->mailer();
    //format the email
    $recipient = $order_email;
    $subject = __("Deine Login-Daten für Geomap", 'theme_name');
    $content = get_custom_email_html( $order, $subject, $mailer );
    $headers = "Content-Type: text/htmlrn";
    //send the email through wordpress
    $mailer->send( $recipient, $subject, $content, $headers );
  }

  function email_exists( $order_email ) {
    $user = get_user_by( 'email', $order_email );
    if ( $user ) {
      send_custom_email($order_email, $order);
//not sure where this variable was comming from
      return $wp_new_user_notification_email;
    }
    else {

      // random password with 12 chars
      $random_password = wp_generate_password();
      // create new user with email as username & newly created pw
      $user_id = wp_create_user( $order_email, $random_password, $order_email );
//not sure where this variable was comming from
      return $wp_new_user_notification_email;
    }
  }



  foreach ( $items as $item_id => $item ) {
    $product_id = $item->get_variation_id() ? $item->get_variation_id() : $item->get_product_id();
    if ( $product_id === XYZ ) {
      //get the user email from the order and check if registered already
      email_exists( $order_email );
    }
  }
}

Now as a conclusion, WP_user class doesn’t give access to the password for security reasons, and they don’t send it by email, you should not either is a very bad practice.

I will suggest you create the account and let WP send the default email and then you send another email with the link to your custom page, but not with the credentials.
That way they will receive two emails one secure and one with the special conditions.