How to override cart/item/default.phtml in Magento 2?

For override default.phtml file in Magento 2, We need to create the plugin for it. We need to override Magento\Checkout\Block\Cart\AbstractCart Block file to override default.phtml in the module.
create the file at the location in your module.

I have created a module with Rbj/CartItem as {Namespace/Modulename},
app/code/Rbj/CartItem/etc/di.xml file,

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <!-- override cart/item/default.phtml file -->
    <type name="Magento\Checkout\Block\Cart\AbstractCart">
        <plugin name="item-test" type="Rbj\CartItem\Plugin\Cart\AbstractCart" sortOrder="1"/>
    </type>
</config>

Create plugin file at below location in your module,
app/code/Rbj/CartItem/Plugin/Cart/AbstractCart.php

<?php
namespace Rbj\CartItem\Plugin\Cart;
class AbstractCart
{
    /*
    *   Override cart/item/default.phtml file
    *   \Magento\Checkout\Block\Cart\AbstractCart $subject
    *   $result
    */
    public function afterGetItemRenderer(\Magento\Checkout\Block\Cart\AbstractCart $subject, $result)
    {
        $result->setTemplate('Rbj_CartItem::cart/item/default.phtml');
        return $result;
    }
}

Create template file in your module view folder,
app/code/Rbj/CartItem/view/frontend/templates/cart/item/default.phtml
Keep your content in default.phtml file to override Cart module default template.