How to Convert Custom Field From Quote Item to Order Item in Magento 2

Step 1: Add a di.xml file
Step 2: Identify a code, a plugin
Step 3: Add a class to your module, then identify a function

Step 1: Add a di.xml file

In the module app/code/Tony/HelloWorld/etc, please add the di.xml file.

Step 2: Identify a code, a plugin

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <type name="Magento\Quote\Model\Quote\Item\ToOrderItem">
        <plugin name="TonyHelloworld_to_order_item" type="Tony\Helloworld\Model\Plugin\Quote\TonyHelloworldToOrderItem"/>
    </type>
</config>

Then, identify a plugin Tony\HelloWorld\Model\Plugin\Quote\TonyHelloworldToOrderItem of class Magento\Quote\Model\Quote\Item\ToOrderItem

Step 3: Add a class to your module, then identify a function

<?php
/**
 * Tony
 *
 * Author : Tony Liu
 * Blog: https://www.tonyblog.cn
 *
 * DISCLAIMER
 *
 * Do not edit or add to this file if you wish to upgrade this extension to newer
 * version in the future.
 *
 * @package Tony_Helloworld
 */
namespace Tony\Helloworld\Model\Plugin\Quote;
class TonyHelloworldToOrderItem
{
    public function aroundConvert(
        \Magento\Quote\Model\Quote\Item\ToOrderItem $subject,
        \Closure $proceed,
        \Magento\Quote\Model\Quote\Item\AbstractItem $item,
        $additional = []
    ) {
        /** @var $orderItem Item */
        $orderItem = $proceed($item, $additional);
        $orderItem->setNotes($item->getNotes());
        return $orderItem;
    }
}

Finally, the result of fuction as follow :

1. Quote notes :

2. Quote to order

3. After create an order

If you want to know more information, please contact me.