Suggestion for GP Premium Database Website Theme that looks like this?

Hi there,

Yeah thats complicated ... are you just after a similar design or will it require the same kind of functionality ?

I'm looking for a right sidebar with collapsible / nested sections - States>Cities, etc.; which brings up the relevant data in card form in the main area. So for example, if the database is setup like the above, selecting Los Angeles brings up the Codes categorized under Los Angeles in what looks like card/kanban form. Might have to build this in an application but thought I'd check and see if there was something pre-built in GP. Thank you!

Without too much work you could create a simple version of that.
The structure would be something like this:

1. Primary ( or Secondary navigation ) location in Left Sidebar.
2. The Menu would be a series of parent and child items. Each child would be a link to a category/sub-category archive.
3. The card style could be achieved either via the Customizer > Layout > Blog options or combined with the GPP 2.0 Block Element that will allow you to design that style in the Block editor - example of that in play:

https://docs.generatepress.com/article/block-element-content-template/

It would require some CSS to fix the sidebar to occupy a fraction of the screen.
But it won't do out of the box is Ajax load the content - so each City selection would effectively navigate to a specific archive.