what is $block->getchildhtml() in topmenu.phml file

I was trying to do/learn something and i am at topmenu.phtml file , i have seen $ block->getchildhtml() there and when i go to block which is namespace Magento\Theme\Block\Html; with the name topmenu.php , i am not seeing any getchildhtml function there . I did research on what getchildhtml function is i have realized it is comming from the layout but when i go to layout which is default.xml file in the module-theme folder there is

<block class="Magento\Framework\View\Element\Template" name="store.menu" group="navigation-sections" template="Magento_Theme::html/container.phtml">                 <arguments>                     <argument name="title" translate="true" xsi:type="string">Menu</argument>                 </arguments>                 <block class="Magento\Theme\Block\Html\Topmenu" name="catalog.topnav" template="Magento_Theme::html/topmenu.phtml" ttl="3600" before="-"/> </block> 

there is no child of this block so from where this getchildhtml will see and print result ?