DDG is a just a re-skin of Bing search. They don't do indexing.
They don't just use bing is my understanding. Unless that's changed?
This was my understanding too. Their Sources page[1] indicates that they run a crawler called DuckDuckBot (implying some indexing takes place), and have community maintained/crowd-sourced data as well.
Those are mostly DDG's instant answers, if I'm not mistaken, where websites use the SDK to submit their data to be crawled.
DDG doesn't have webmaster tools or crawls domains natively.
They also use Yandex, which isn't very relevant unless your website is in Russian, and allow websites to manually set up their pages for instant answers.