By paying for API access to other search engines. Yahoo used to offer that publicly but eventually shut the service down (but kept DDG as a legacy customer)
Approximately $1 per 1000 requests https://www.programmableweb.com/news/yahoos-new-search-api-pricing-compared-to-google-bing/analysis/2014/11/24 https://www.programmableweb.com/news/yahoos-new-search-api-p...