How can i customize the thank you or order confirmation page?

The thank you or order confirmation page can be customized in different ways, if you want to do without plugins altogether.

1st variant: Via redirection to a separate subpage

add_action( 'template_redirect', 'woo_custom_redirect_after_purchase' );
function woo_custom_redirect_after_purchase() {
  global $wp;
  if ( is_checkout() && !empty( $wp->query_vars['order-received'] ) ) {
    wp_redirect( 'https://www.zurgewuenschtenseite.de' );
    exit;
  }
}

2nd variant: Customize the Woocommerce page

// Change the heading

add_filter( 'the_title', 'woo_title_order_received', 10, 2 );
function woo_title_order_received( $title, $id ) {
  if ( function_exists( 'is_order_received_page' ) && 
       is_order_received_page() && get_the_ID() === $id ) {
    $title = "Vielen Dank für Ihre Bestellung und alles andere";
  }
  return $title;
}

// Change the second heading

add_filter('woocommerce_thankyou_order_received_text', 'woo_change_order_received_text', 10, 2 );

function woo_change_order_received_text( $str, $order ) {
    $new_str = 'Dies steht nun in der zweiten Zeile';
    return $new_str;
}

// Forwarding to a separate subpage for a specific product (based on the product ID)

add_action( 'woocommerce_thankyou', 'redirect_product_based', 1 ); 
function redirect_product_based ( $order_id ){
            $order = wc_get_order( $order_id );
            foreach( $order->get_items() as $item ) {
                if ( $item['product_id'] == 643 ) {
                     wp_redirect( 'https://www.weiterleitungzurunterseite' );
                
                }
            }        
}