magento 2 how to add phtml file after after_body_start

You can check inside page_layout/empty.xml file inside module-theme/base folder.

<container name="after.body.start" as="after.body.start" before="-" label="Page Top"/>

<container name="before.body.end" as="before_body_end" after="-" label="Page Bottom"/>

You can check reference from vendor/magento/module-catalog/view/frontend/layout/default.xml here also:

<referenceContainer name="after.body.start">
    <block class="Magento\Framework\View\Element\Js\Components" name="head.components" as="components" template="Magento_Catalog::js/components.phtml"/>
</referenceContainer>

So in Magento 2 , you can use referenceContainer node to define same as in Magento 1. Thanks.