Hmm, First thing it asks you is to access you data and browser info.
That is on your device (phone/computer). There is a fire logo which you can tap which clears the browser history on your device. DDG say they don't store your history on their servers like many others.
I've been using DDG for quite a while, it is good, but doesn't always get everything. If I don't find quite what I am looking for, for example a part, I'll try Google with the best info I have and see if it throws up any other suggestions. Google gets some fairly unrelated search data, but don't get to constantly track my location because of my phone.
Ever looked up a company on Google and found it shows you a graph of their busiest/quietest time? that is because they track how many phones are there...
If it is free, then you are the product...!