Hi
Thanks for contacting us. You can follow this tutorial for megamenu
https://www.youtube.com/watch?v=DAme45jfoa0&t=2s
And for installing Helix Ultimate, you have two options. You can install the template only OR you can install the quikcstart pack. The QuickStart pack is a complete demo website that includes:
The full Joomla CMS package
Components, modules, and templates
All necessary data and configurations
All these files are zipped together, making the package larger in size. Installing the QuickStart pack will replicate the template exactly as shown in the demo, providing a fully functional website setup.
For step-by-step guidance on installing the QuickStart pack, please refer to this article:
https://www.joomshaper.com/blog/how-to-install-quickstart-package-in-joomla-4
https://www.joomshaper.com/blog/how-to-install-a-joomla-template
Hope these links will help you.
-Regards.