searx and searxng are not search engines, and searx is more private (searxng collects info from users, which searx never wanted to). AFAIK duckduckgo is neither a search engine on its own, it uses blink...
To tell the truth, only yacy and sightnet have own index(database).
Others are meta-search engines. They work like proxy. It's called meta search. (https://en.wikipedia.org/wiki/Metasearch_engine)
Just looked it up since I was sure I had read they had their own. On their wikipedia article it says:
In its early days on the Internet, the Qwant search engine relied on Bing to provide more relevant results. In 2016, Qwant claimed to be increasingly using its own results from its own exploration robots. It is still at the status of hybrid engine.[89] In 2020, Qwant claimed to have exceeded 50% of independent results for web searches, and 70% for all researchs