To overwrite any adminhtml template file you need to create custom module. For you order item section changes I have overwrite vendor/magento/module-sales/view/adminhtml/templates/order/view/items/renderer/default.phtml in to my module.
First of all create etc/module.xml file. Below is the code for module.xml file.
<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Module/etc/module.xsd">
<module name="Darsh_Orderitem" setup_version="1.0.0" >
</module>
</config>
Create view/adminhtml/layout/sales_order_view.xml file in your module. In this file I have used "referenceBlock" tag to overwrite core template file and used "setTemplate" method.
<?xml version="1.0"?>
<!--
/**
* Copyright © Magento, Inc. All rights reserved.
* See COPYING.txt for license details.
*/
-->
<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" layout="admin-2columns-left" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
<body>
<referenceBlock name="default_order_items_renderer">
<action method="setTemplate">
<argument name="template" translate="true" xsi:type="string">Darsh_Orderitem::order/view/items/renderer/default.phtml</argument>
</action>
</referenceBlock>
</body>
</page>
Now, create phtml file in your folder view/adminhtml/templates/order/view/items/renderer/default.phtml
<?php
/**
* Copyright © Magento, Inc. All rights reserved.
* See COPYING.txt for license details.
*/
// @codingStandardsIgnoreFile
?>
<?php /** @var \Magento\Sales\Block\Adminhtml\Order\View\Items\Renderer\DefaultRenderer $block */ ?>
<?php $_item = $block->getItem() ?>
<?php
$objectManager = \Magento\Framework\App\ObjectManager::getInstance();
$product = $objectManager->get('\Magento\Catalog\Model\ProductFactory')->create()->load($_item->getProductId());
?>
<?php $block->setPriceDataObject($_item) ?>
<tr>
<?php $i = 0;
$columns = $block->getColumns();
$lastItemNumber = count($columns) ?>
<?php foreach ($columns as $columnName => $columnClass):?>
<?php $i++; ?>
<td class="<?= /* @noEscape */ $columnClass ?><?= /* @noEscape */ ($i === $lastItemNumber ? ' last' : '') ?>"><?= /* @escapeNotVerified */ $block->getColumnHtml($_item, $columnName) ?>
<?php if($columnClass == "col-product"){ ?>
<span><?php echo __("PCB:") ?></span>
<?php echo $product->getData('pcb_master');; ?>
<?php } ?>
</td>
<?php endforeach; ?>
</tr>
In phtml file I have load product data and get "pcb_master" attribute value.
If you still you are facing issue then you can download module from https://github.com/Darshanmodi1427/Orderitem
Thanks.