Magento – Magento 2: change template for block in “sales.email.order.renderers”

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())) ?>

Suppose package:Learning and module: Custom

Module.xml looks like

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../../../../../lib/internal/Magento/Framework/Module/etc/module.xsd">
    <module name="Learning_Custom" setup_version="2.0.0">
        <sequence>
            <module name="Magento_Catalog"/>
        </sequence>
    </module>
</config>

And then create a layout xml following path -> app/code/Learning/Custom/view/frontend/layout/catalog_product_prices.xml

<layout xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../../../../../../../lib/internal/Magento/Framework/View/Layout/etc/layout_generic.xsd">
    <referenceBlock name="render.product.prices">
        <arguments>
            <argument name="default" xsi:type="array">
                <item name="prices" xsi:type="array">
                    <item name="tier_price" xsi:type="array">
                        <item name="render_template" xsi:type="string">Learning_Custom::product/price/tier_prices.phtml
                    </item>
                </item>
            </argument>
        </arguments>
    </referenceBlock>
</layout>

Create template inside your module or copy base template file inside your module path should be -> app/code/Learning/Custom/view/frontend/templates/product/price/tier_prices.phtml