Index of Types -> Domains

I've seen this come up a bit in the SEO community where webmasters and
researchers are trying to look for examples of domains implementing a
particular Type or format in their pages.

Moreover, when I look at the documentation for any given type--Product
<http://schema.org/Product>, for example--I see a note about its usage: *Usage:
Over 1,000,000 domains*

How is this calculated, where is stored, and how might I make use of it?
Has an index of Types to Domains already been built?

If not, has anybody explored building one? At the outset, I imaging
something complementing the aforementioned Type documentation. For each
type, I want to see what sites include that in their schema.org markup, and
what implementation they use (microdata vs. json+ld).

If this has already been built, I'd love to make use of it for my own
learning. If it hasn't...well, I suppose I might start working on building
it.