Magento 2 PHTML – How to Call Default Menu

With this code you create this block and render it immediately, without adding it to the layout hierarchy. That means it doesn't have access to the parent block because there is no parent.

To create a block dynamically as child of the current block, use:

$child = $this->getLayout()->createBlock("Magento\Framework\View\Element\Template")->setTemplate("Namespace_Module::state.phtml");
$this->addChild('state', $child);

And to render it:

echo $this->getChildHtml('state');

(I used "state" as alias here, but it can be anything else)

Now you have access to the parent from your state.phtml template, using $this->getParentBlock()

If I set state.phtml in country.phtml like

<?php
echo $this->getLayout()->createBlock("Namespace\Module\Block\Country")->setTemplate("Namespace_Module::state.phtml")->toHtml();
?>

Then Is it possible to call functions of Country.php in State.phtml file ?

If you insist, you can do it like this:

 echo $this->getLayout()
     ->createBlock("Namespace\Module\Block\Country")
     ->setTemplate("Namespace_Module::state.phtml")
     ->setData('country_block', $this)
     ->toHtml();

And then, in state.phtml:

$this->getData('country_block')->METHOD_OF_COUNTRY_BLOCK();

{{block class="Magento\Theme\Block\Html\Topmenu" name="catalog.topnav" template="Magento_Theme::html/topmenu.phtml"}}