Depends, some of them, like Startpage (proprietary) or Whoogle (FOSS) use the Google engine (Whoogle without the ads), others are using Bing, some metasearch engines like Metager use several engines and a own one, Andi is a special case, it don’t show a lot of results, but the most relevant ones corresponding to your cuestion, or shows a direct answer. It’s a AI driven search assistant who understands the concept of your search (It’s still in beta, but in heavy developement and works very well, I use it in first instance when Isearch specific things. Anyway itself permits the search with several other engines. What I like most, also permits to view most pages in the result in Reader view, without the need to open them.