DDG gets results back from bing, sometimes modifying it with it's own results to make them better.
> But I think you're talking about which index it is that DDG queries internally to yield regular (non-bang) search results?
Yes. Most of it is Bing.
Ecosia doesn't proxy it properly. It sends it to bing from you browser, not from Ecosia's servers. Bing knows your IP and sets cookies to track you. You can observe it with dev tools.
They might have changed it since I last checked.
If you are looking for proxied Google results, your main options are Runnaroo [0] and Startpage [1].
I'm the creator of Runnaroo, and I prefer it to Startpage (and also DDG, but that is another thread).