It at one point used Google's index... I guess it wasn't directly but indirectly... I just remember hearing that they used Google's index and never looked into it
https://money.cnn.com/2009/07/29/technology/microsoft_yahoo/
https://www.clrn.org/why-does-google-keep-switching-to-yahoo/
Unless I'm wrong... The first one says that Yahoo and Microsoft worked together and Microsoft used Yahoo's search results and such... And Yahoo at around the same time also used Google's search index to help power their searches....
I also didn't look that hard and might of missed something but