How do I include data from the order item type fields in the order receipt email?
Figured it out! Took a few steps:


I created a View of type: Order Item. Added my custom fields to that view (Job Title, etc).
Added a Contextual Filter "Order item: ID".
Provide Default Value: Content ID from URL.
I created a Viewfield on
each of my Order Item Types, using the View I created in the
previous step.
I went to
/admin/commerce/config/order-item-types/default/edit/display and hid
all the fields except for the Viewfield. Repeat for the other Order
Item Types. In commerce-order-receipt.html.twig, add this code
(which was found in commerce-checkout-order-summary.html.twig):

{{ order_item|commerce_entity_render('summary') }}

Style the output as you like in the receipt twig. Here is the
pertinent portion of mine in case anyone finds it helpful:

        &lt;tr&gt;
          &lt;td&gt;
            {% block order_items %}
            &lt;table style="padding-top: 15px; padding-bottom:15px; width: 100%"&gt;
              &lt;tbody style="text-align: left;"&gt;
              {% for order_item in order_entity.getItems %}
              &lt;tr&gt;
                &lt;td&gt;
                  &lt;span style="font-size: 15px;"&gt;
            {{ order_item.getQuantity|number_format }} x 
            &lt;span style="font-weight: bold;"&gt;{{ order_item.label }}&lt;/span&gt;
            &lt;span style="float: right;"&gt;{{ order_item.getTotalPrice|commerce_price_format }}&lt;/span&gt;
          &lt;/span&gt;
          &lt;br&gt;
          {{ order_item|commerce_entity_render('summary') }}
          &lt;br&gt;&lt;br&gt;&lt;hr&gt;&lt;br&gt;
                &lt;/td&gt;
              &lt;/tr&gt;
              {% endfor %}
              &lt;/tbody&gt;
            &lt;/table&gt;
            {% endblock %}
          &lt;/td&gt;
        &lt;/tr&gt;

Clear cache, and that's it!