How technical would an AC and interview be for a summer internship in S&T? What do you recommend me researching?

Normally ACs are not very technical and focus more on soft skills and transferable skills which will be relevant to your role. There were a few tasks that I had to complete as part of my experience day, though as I did it during COVID I didn't have a group task. The first was a maths assessment which wasn’t very long, just to verify any tests that you’d done earlier in the application process. Then, there was a case study activity with a few different resources which you had to answer some questions about, for example to give solutions. There was another individual task relating to time management/task priority management. The last was a short interview which was quite informal, more of a discussion about your strengths, experience, motivation, why you want to work for EY, etc. Just the standard questions, nothing technical. I would just suggest researching EY and write down a few notes about what you could say in the interview specifically about EY and your role as well as your own experience, strengths and skills. With the group task, I would suggest to ensure you’re contributing well independently but also collaborating efficiently with others. I did have this 3 years ago so some things probably have changed, but I hope this is useful. Best of luck!