In the diagram below, each second level menu item (header) forms a column, shown in grey (the "box" ).
The menu item consists of two parts: the Item Content (link, image, icon, description), and the submenu (child items)
The Grid Layout determines the size of the grey box
The Item Content Layout determines the arrangement of items inside the pink Item Content box
The Grid Layout determines the size of the menu item column relative to its parent container. In the diagrams, this simply means the width of the grey column boxes relative to the blue outline (submenu)
The setting is found in the Menu Item Settings > Layout > Columns Width
The Item Layout setting controls the arrangement of the elements inside the item, not including the submenu. It is represented by the pink box in the diagram below
Elements that make up the Item Content include the Title, Description, Image, Icon, etc. Here are a few examples, zoomed into the target level:
These layouts can be selected via the Menu Item Settings > Layout > Item Layout
The layout can be configured like this