Example. Top menu:
Comment manually children category in topmenu.phtml:
<?php /* @escapeNotVerified */ /*echo $block->getChildHtml();*/ ?>
After click or hover - uncomment this code:
<?php /* @escapeNotVerified */ echo $block->getChildHtml(); ?>
What is the best solution? I want to reduce the number of links in the body of html, but I don't want to hide the top menu (I believe that this should be implemented in the basic functionality of the Magento).
Php code is executed on the server side and send to the browser. Hovering is done inside the browser and does not hit the server. Clicking might if there is additional request. If you want to show additional menu after hovering/clicking you need to render it in html, set display:none on it and prepare event handler in javascript to remove toggle that style when the action happens.
[EDIT]
If you want to load additional part later you have 2 options:
1) Create an ajax request on clicking/hovering and prepare rest endpoint or a controller that will return the requested part and then inject it to the DOM.
2) If the above will be too slow in execution you can instead of building actual menu create a json array with links and build the menu on the fly with js when user wants to use the menu.
Try to remove this block from your theme page, for example: catalog/product/view (catalog_product_view.xml).
Insert this peace of code on your customised XML.
<referenceBlock name="catalog.topnav" remove="true"/>
