The process is extremely transparent from the onset with respect to the stages and the general length of the process. They can speed up (but not skip) review stages if you have another offer on hand, as was the case with me. Furthermore, there is compensation for each stage of the process (which is not common), and there is no expectation of live-coding.
As DuckDuckGo is remote-first, there is obviously a high focus on your ability to communicate. There is also a focus on your ability to self-manage, as you will be working independently. Coding and design skills are validated towards the end of the process rather than the beginning, which is different to other tech firms.
There were 7 stages in total over approximately 4 months: 1. initial TA discussion, 2. take home written assignment, 3. interview with engineers, 4. take home design assignment, 5. review with engineers, 6. coding submission, 7. final rounds with senior stakeholders (including the CTO). All of this is laid out on initial contact.
For me, I rated the process as "difficult" primarily due to my own time management constraints: I was in final stages with one other firm, working full-time, along with other commitments outside of work. The projects were not PhD-level-algo difficult. But they were interesting, and took time and consideration which I needed to plan for in an already-busy schedule.
So the advice I'd offer to future candidates is to allow ample time in your schedule, polish up your written comms, and reflect on your experience to date. And to set expectations: this is not your typical tech-firm interview. That is, cramming leetcode will not be useful for passing this process.