This functionality will be done by and event observer. And Event is “checkout_onepage_controller_success_action”.

In this event you can get the current order data and you will be able to export this data in to csv.

Step 1: For add the event you have to make a events.xml in your module and paste the below code into it.

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
    <event name="checkout_onepage_controller_success_action">
        <observer name="customorderexport" instance="MagemonkeysCustomexportObserverOrderexport" />
    </event>
</config>

Step 2: Now you have to create a Orderexport.php file in Observer folder and place the below code into it.

<?php

namespace MagemonkeysCustomexportObserver;

use MagentoFrameworkEventObserverInterface;
use MagentoFrameworkAppFilesystemDirectoryList;
use MagentoFrameworkFilesystem;
use MagentoFrameworkFilesystemDirectoryWriteInterface;
use MagentoStoreModelScopeInterface;

class Orderexport implements ObserverInterface
{
	protected $_request;
	protected $_order;
	protected $_productRepository;
	protected $_scopeConfig;
	protected $_customer;
	protected $_storemanager;

	public function __construct(
	    MagentoFrameworkAppRequestInterface $request,
	    MagentoSalesModelOrder $order,
     	MagentoFrameworkAppResponseHttpFileFactory $fileFactory,
		Filesystem $filesystem,
        MagentoFrameworkAppConfigScopeConfigInterface $scopeConfig,
        MagentoCustomerModelCustomerFactory $customer,
    	MagentoStoreModelStoreManagerInterface $storemanager,
		MagentoCatalogModelProductRepository $productRepository
	) { 
		$this->_scopeConfig = $scopeConfig;
		$this->_customer = $customer;
		$this->_storemanager = $storemanager;
	    $this->_request = $request;
	    $this->_order = $order;
	   	$this->_fileFactory = $fileFactory;
        $this->directory = $filesystem->getDirectoryWrite(DirectoryList::VAR_DIR);
        $this->_productRepository = $productRepository;
 	}

	public function execute(MagentoFrameworkEventObserver $observer) {
		
		$post = $this->_request->getPost();
		
		$orderids = $observer->getEvent()->getOrderIds();


	  	foreach($orderids as $orderid){

        	$order = $this->_order->load($orderId);


        	$websiteID = $this->_storemanager->getStore()->getWebsiteId();
			$customer = $this->_customer->create()->setWebsiteId($websiteID)->loadByEmail($order->getCustomerEmail());
        	$customerId = $customer->getId(); // using this 
        	$customerPoNumber = $customer->getCustomerNumber(); // using this 


			$headers = array('Magento_Account_Number','Back_Office_Account_Number','Transaction_Date','Transaction_Time','Magento_Transaction_Number','Client_Reference_Number','Item_Number','Quantity_Sold','Selling_Price','Discount');

			$name = strtotime("now");
		    $file = 'customorderexport/'.$name.'_detailed_orderexport.csv';
		    $this->directory->create('customorderexport');
		    $stream = $this->directory->openFile($file, 'w+');
		    $stream->lock();
		    $stream->writeCsv($headers);
		    
	        

	        $items = $order->getAllItems();
			foreach ($items as $item) {
				$orderdetail['Magento_Account_Number'] = $orderdata->getData('created_at');
		        $orderdetail['Order_Number'] = $orderdata->getData('increment_id');
		        $orderdetail['Transaction_Date'] = date('Y-m-d',strtotime($order->getCreatedAt()));
		        $orderdetail['Transaction_Time'] = date('h:i A',strtotime($order->getCreatedAt()));
		        $orderdetail['Magento_Transaction_Number'] = $order->getIncrementId();
		        $orderdetail['Client_Reference_Number'] = $customerPoNumber;
				$id = $item->getProdcutId();
				$product = $this->_productRepository->getById($id);
		        $orderdetail['Item_Number'] = $product->getAutoKoolNumber();
		        $orderdetail['Quantity_Sold'] = $item->getQtyOrdered();
		        $orderdetail['Selling_Price'] = $item->getPrice();
	        	$orderdetail['Discount'] = $order->getDiscountAmount();
	        	$stream->writeCsv($orderdetail);
		    }

		    $stream->unlock();
		    $stream->close();
        }
	}

}
?>

That’s it.

Note- I have use the field and data as per my requirements. You will change this as per your requirements.