Newbie question - different menu on different platforms

Im sure there is an obvious answer but I cant find it.
My ecommerce website (built from a template) has a landscape menu on the top of the page. It is populated using the categories from the ecommerce products. On a smartphone its too wide, but if it is vertical it obscures the products below, so its ugly.
I want to make it a drop down for phones.
If I turn it off for phones and make a new one, Im no good at formatting a new menu, and I cant figure out how to reuse all the elements within. If I duplicate them into the new menu and make changes, those changes affect the other versions. There is no unlink option on the top level element, and even if there was all the other bits will still be shared.

Here is my site Read-Only:

(how to share your site Read-Only link)

In the designer you have 4 screen sizes (breakpoints) to design the website.
You can switch between the sizes from the top nav bar in the designer mode.

When you select landscape or portrait, you can design and move the section down from the menu button. This will not change the layout on pc screen size.

Here’s a video that explains it well.