Magento\Checkout\Model\PaymentInformationManagement
public function savePaymentInformationAndPlaceOrder(
$cartId,
\Magento\Quote\Api\Data\PaymentInterface $paymentMethod,
\Magento\Quote\Api\Data\AddressInterface $billingAddress = null
)
The billingAddress is passed as second argument.
Chris, Magento Developer, https://www.chrisranjana.com