Be calm but keen when delivering your responses; nerves are not a good look, but neither is bored disinterest. Prepare by reading the company website and knowing the products and how they work together.
Know the basics of your field really well, and be prepared to talk about something you have worked on in as much detail as you can. Showing a good understanding of the basics and also that you can dive into a topic to learn about it when you need to.
Be enthusiastic. Show your desire to help people. Demonstrate your leadership capabilities. Show you are collaborative.
Prior diverse experience (work, university, extra-curricular). Confidence and interpersonal skills. Alignment to company values
For the summer student interviews (which is a process I didn't go through), I'd definitely recommend talking about extra-curricular activities, and how they taught you something or how they show you can approach difficult problems or situations.