• From handle="sales_email_order_items"

{{layout handle="sales_email_order_items" order_id=$order_id area="frontend"}

• you'll see this layout "sales_email_order_items"

<block class="Magento\Sales\Block\Order\Email\Items" name="items" template="Magento_Sales::email/items.phtml" cacheable="false">
<block class="Magento\Sales\Block\Order\Totals" name="order_totals" template="Magento_Sales::order/totals.phtml">
</block>

Maybe, you'll find it in file

vendor/magento/module-sales/view/frontend/templates/email/items.phtml

or

vendor/magento/module-sales/view/frontend/templates/order/items.phtml

have a nice day !!