Magento – Override new order email in magento 2

When passing a variable this way:

{{layout handle="sales_email_order_items" order=$order  test=$test}}

You make it available in the email/order/items.phtml template only.
You are able to get it there with: $this->getTest() or $this->getData('test').
To pass it further to the email/order/items/order/default.phtml edit the items.phtml file and add this

$item->setTest($this->getTest());

before calling

<?php echo $this->getItemHtml($_item) ?>

To make it shorter, replace:

<?php echo $this->getItemHtml($_item) ?>

with

<?php echo $this->getItemHtml($_item->setTest($this->getTest())) ?>

Depending on what kind of data you want to add, you might want to use an observer instead of rewriting the model:

etc/events.xml

<?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="email_order_set_template_vars_before">
        <observer name="add_extra_data_to_transport" instance="Vendor\Module\Observer\AddExtraDataToTransport" />
    </event>
</config>

Then create Observer\AddExtraDataToTransport.php

<?php

namespace Vendor\Module\Observer;

use Magento\Framework\Event\ObserverInterface;

class AddExtraDataToTransport implements ObserverInterface
{  
    public function execute(\Magento\Framework\Event\Observer $observer)
    {
        $transport = $observer->getEvent()->getTransport();
        $transport['test'] = '<p> Yoo Yeezy </p>';
        if ($transport['payment_html'] == 'transfer') {
            $transport['payment_html'] = 'test';
        }
    }
}