@Jeffrey -- not quite the graphic you were looking for, but here's my 2 cents and hopefully this helps to move the discussion forward:
Google Home: these are pulled most from featured snippets in Google search results and also Google Maps / GMB's for locally targeted searches. You can see / hear a direct correlation between featured snippets and voice results.
Alexa: Amazon (obviously) for product and Amazon shopping related data, a combination of Bing for direct search results and queries, and Yelp (via Apple Maps) for local business related searches.
Siri: a combination of Apple Maps and Yelp, but Siri also relies heavily on Google search results.
Cortana: web search results from Bing and also Bing's Knowledge Graph
All, or most... or all, of these platforms leverage (or look for) Schema markup for different types of content (@types).
But I definitely look forward to other, more "expert knowledge" feedback as this is an excellent question!