https://github.com/magento/magento2/blob/2ea8cdd7/app/code/Magento/Checkout/Model/GuestPaymentInformationManagement.php#L73-L86
See also:
\Magento\Sales\Model\AdminOrder\Create::createOrder()
\Magento\Quote\Model\QuoteManagement::submit()
\Magento\Quote\Api\CartManagementInterface::placeOrder()