[hobofan]

Google mines that data by other people searching for that, not (only) by scraping it off sites. Yes, they also do that, but AFAIK search trends have a higher relevance than scraped suggestions.

As long as people want to know how their existing product stacks up against another product they have heard of before, "<existing product> vs <other products>", will be searched and appear in the autocomplete.

This approach breaks down tough for newer products, ot if you want to find the best product by quality, not just popularity.