A mega menu is an expanded navigation menu that displays multiple categories, collections, products, or banners within a large dropdown, rather than a simple menu. This helps customers easily find product categories, clearly shows multiple menu levels, and improves navigation and product discovery.

To add a mega menu, in the Theme Editor on the left, select Header, then select Add Block -> Mega Menu. Then, in the Top-level Menu item field, you need to enter the exact name of the menu you created in your Shopify Admin.

Radian is designed as a Mega Menu with high flexibility for customization, where the way columns are displayed is designed to be easily customized.
Take a look at the image below: the default settings use Columns as 3, and Nested Columns as 2.

How does it work, then? Suppose we have 1 menu and 2 promo images (banners). When you set Columns to 3, it distributes them evenly across all 3 components, as shown in the image below.

In case there is still 1 menu and 2 promo images, but if you set Columns to 2, Radian will prioritize displaying the Menu and remove 1 of the second promo images as shown in the image below

If you have 1 Menu and 2 promo images, but set Columns to 4, it will display as follows

WHAT IS NESTED COLUMNS?

Similar to Columns, Nested columns apply only to the Menu columns. That is, if your menu has a hierarchical submenu, it displays in each column instead of being laid out vertically in a single column
Reference image below:

Nested Columns: 1

Nested Columns: 2

Nested Columns: 3

As you can see in the image below, when Nested columns is set to 3 but my menu only has 2 submenus, it will create empty space in the third column