Please find below steps to override default.phtml by using plugin

1. Create an di.xml in : /app/code/Vendor/ModuleName/etc

<?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="MagentoCheckoutBlockCartAbstractCart">
        <plugin name="change_template_for_default" type="VendorModuleNamePluginCartAbstractCart" sortOrder="1"/>
    </type>    
</config>

2. Create an AbstractCart.php in : app/code/Vendor/ModuleName/Plugin/Cart

<?php
namespace VendorModuleNamePluginCart;
class AbstractCart
{
    /*
    *   Override cart/item/default.phtml file
    *   MagentoCheckoutBlockCartAbstractCart $subject
    *   $result
    */
    public function afterGetItemRenderer(MagentoCheckoutBlockCartAbstractCart $subject, $result)
    {
        $result->setTemplate('Vendor_ModuleName::cart/item/default.phtml');
        return $result;
    }
} ?>

You can change below file as per requirement your custom module base

3. Create an default.phtml in : /app/code/Vendor/ModuleName/view/frontend/templates/cart/item