hide/show product subcategories dropdown menu

That is a basic select menu that you can code in your sidebar or have a search here: https://www.ads-software.com/plugins/search/select+menu/.

it doesn’t look so ‘basic’
i mean,

sub-menu shows on click in first menu? is this hard to do or not?

Sorry, after second look it seems a bit more complicated. You may need the help of a developer.

Are the menu options category based?