The sad thing is that AOL & Yahoo is really Bing, and Ask is I believe Google. So essentially it comes down to Google and Bing.
To get DDG you'll need to use a different browser like FF.
I actually blocked Chrome on my phone and started using FF once I noticed that Chrome was updating itself even though I was requiring manual updates. Play Store and Google Play Services do the same thing, but don't have a good alternative for them,
Yahoo was using Bing from around 2010 to 2015. But since then they've been using a variety of sources - Bing, Google, and presumably their own, and I think it can mix the results.
Yahoo's search business was always odd... They had their own search, then switched to search powered by Google. Then they purchased Overture, which owned AlltheWeb and AltaVista. Overture had originally purchased AlltheWeb from FAST, who had pivoted into the enterprise space, and later got bought by Microsoft and integrated into SharePoint.
AOL has Bing in the footer, Yahoo had Bing references in the source view when I checked, although it looks like what tallanvor said and they just return mix of search engines.
To get DDG you'll need to use a different browser like FF.
I actually blocked Chrome on my phone and started using FF once I noticed that Chrome was updating itself even though I was requiring manual updates. Play Store and Google Play Services do the same thing, but don't have a good alternative for them,