Using a Google Map with an overlay showing boundaries and interact with it

Hi everyone.

I’ve been asked to put a Google Map onto a webpage which has an overlay showing county regions. When a region is selected on the page (could be a button or select dropdown) or they click on the actual map, we filter the data so relevant information is shown.

See this as an example:

I can create the map in Google Maps, I can import the KML file to provide all the boundaries but I don’t know how to interact with it when embedded on the page.

Has anyone got experience with this?