A lot of people saying this is a Chromium fork, but According to their blog, that's not fully the case.
What's under the hood
DuckDuckGo for Windows was built with your privacy, security, and ease of use in mind. It’s not a “fork” of any other browser code; all the code, from tab and bookmark management to our new tab page to our password manager, is written by our own engineers. For web page rendering, the browser uses the underlying operating system rendering API. (In this case, it's a Windows WebView2 call that utilizes the Blink rendering engine underneath.)
So it uses Window's default web renderer. It just happens that the default renderer for windows is the same as Chromium's renderer. In this way it does mean that it has a lot of the same problems as Chromium forks, but the browser itself isn't a chromium fork. (This is also why the macOS version uses Webkit and not Blink.)
A lot of people saying this is a Chromium fork, but According to their blog, that's not fully the case.
So it uses Window's default web renderer. It just happens that the default renderer for windows is the same as Chromium's renderer. In this way it does mean that it has a lot of the same problems as Chromium forks, but the browser itself isn't a chromium fork. (This is also why the macOS version uses Webkit and not Blink.)