Comment by dclara

Because to my understanding, 2-tiered search means that the routing search engine scrapes data from the second level search engines and return it to the users. The second level search engines, e.g., Stack Overflow, are usually running by separate entities from the routing search engine, say DDG. If DDG does not own all the second level search engines, how can they get the local indexes and ranking algorithms from them? And even if DDG does get it, it's no longer decentralized any more. So what's the difference from Google?