navigation Help with horizontal drop down menu running width of the screen

Hello. Loving the drop down menu that runs horizontally full screen with borders around the links.
Does anyone know how to do this or what it would be called?

clearygottlieb(dot)com/

Sounds like you are looking for a "mega menu", there are lots of plugins for this, here's one I've used successfully:

https://www.will-myers.com/products/p/mega-menu-for-squarespace-71

Thank you, diving in now.