That might be an apt description of 3rd tier clickbait 'yellowpages' sites. But the more prominent ones spend large sums of money buying and aggregating data feeds.
3 karma · joined August 4, 2011
And the closest thing to open source, turnkey search is gluing together Apache SOLR and a web crawler. Lucid Imagination offers this (plus other features) as a commercial product, but it not open source to the best of my knowledge.