woocommerce add button underneath order details

You can write the below code to your active theme’s functions.php to achieve this:

// define the woocommerce_before_checkout_billing_form callback 
function action_woocommerce_before_checkout_billing_form( $wccs_custom_checkout_field_pro ) { 
    // Write your button code or any html here.... 
}; 

// add the action 
add_action( 'woocommerce_before_checkout_billing_form', 'action_woocommerce_before_checkout_billing_form', 10, 1 );