https://www.startpage.com/privacy-please/startpage-articles/enhanced-search-results-on-startpage

https://support.startpage.com/hc/en-us/articles/4522435533844-What-is-the-relationship-between-Startpage-and-your-search-partners-like-Google-and-Microsoft-Bing

Looks like they started mixing in results from Bing a couple of years ago. They're a bit vague on who if anyone is in the "others" category