Please use the following code snippet. It’ll add a new button right after the add to cart button. The code is well commented, hope that’ll be helpful for you.
add_action( 'woocommerce_after_add_to_cart_button', function() {
/**
* If you need product data here you can set $product as global
* then you can easily access product data through $product object
*
* global $product;
*/
/**
* You may have to build product customization link automatically
* if the customization link follows same pattern and has product
* id then you can store the customization product id in product meta
* and retrive that meta using $product->get_meta( '_customization_id' );
*/
$builder_product_link = 'http://builder.alltype.com/';
?>
<a class="button" href="https://wordpress.stackexchange.com/questions/296423/<?php echo esc_url( $builder_product_link ); ?>"><?php esc_html_e( 'Customize Product', 'text-domain' ); ?></a>
<?php
} );