The easiest way to create nested menus in Shopify is to buy a theme from the Shopify Theme store that has nested menus built into the theme.
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Remember is you already have a Shopify website, changing or Publishing a new theme to you website will loses the any customization from your previous theme.
Here are a couple of examples of multilevel sub menus on websites
The first is the Vanity Theme which enables pop out menus. A link to this Shopify Theme is at the end of this Blog.
The next example is the Editions Theme which has subcategory choices.
These are just a couple of examples to see more sub menu options check out the links at the end of this article.
If you want a powerful e-commerce platform that is easy to use, Shopify offers a tremendous platform.
Here is a list of themes that include the ability to do a multiple level submenu or pop out sub menu.