It's all about technical questions! Focus on learning the ins and outs of C++ since they focus mostly on it. I studied too deeply the company and what projects they worked on but they didn't ask anything like that! They will also ask you about your CV and explain a few projects you've worked on. It might change on your interview so the only tip I could give you is to study the language you know well deeply.

The assessment/interview day has candidates for all placement role. They do a brief introduction of the company who uses their product and so on. You get to ask question regarding the company. Then there is a site viewing where you get to see where you might be stationed and what the current placement employee are doing. After that, all the candidates go through the assessment which is same for every candidate on that day. Then you get to speak with current placement and graduate employee. After that you will be interviewed by two of their senior engineer or HR representative( that was for me). They mainly asked me technical questions regarding the role. Questions might be related to what you put in the CV.