
According to The Verge, the one billion figure comes from products that are listed via Google Shopping:
The 1 billion items figure comes from products available through Google Shopping, so it likely doesn’t include more obscure, unshoppable objects, such as a gaming console from the 1990s or the first edition of a rare book. But it covers a huge range of things that could appease someone who’s simply just looking up an item they’re curious about.