how to create additional button on single product page

Question

To achieve something like this you should change the woocommerce_after_shop_loop_item hook, which is where the add to cart button is added.

To replace it with custom behavior, first, you remove the button, then add your custom button in its place.

add_action( 'init', 'remove_add_to_cart_button' );
function remove_add_to_cart_button() {
    remove_action( 'woocommerce_after_shop_loop_item', 'woocommerce_template_loop_add_to_cart', 10 );
}

add_action( 'woocommerce_after_shop_loop_item', 'new_add_to_cart_button' );
function new_add_to_cart_button() {
    // Your button code.
}

You can find more about WooCommerce hooks here: https://docs.woocommerce.com/wc-apidocs/hook-docs.html.