How to Create Accordion Sidebar Menu

Hi there,

The example site is using a WP core Navigation Menu widget in the left sidebar:
https://wordpress.com/support/widgets/navigation-menu-widget/

That being said, it will require a complete custom CSS solution to replicate that style and this isn't something we can support here:
https://generatepress.com/what-support-includes/

I will mention that the example site isn't using any caching plugin so the source code isn't hidden - this means that you can try to use the browser inspect tool to see the potential CSS involved.

Hope this helps :)