How to answer "what does an investment banker do?"

This one comes down to how junior you're talking; the below would apply to analysts.

Here's the context: at the highest level investment banks do two things, raise capital and advise on "strategic alternatives". Depending on the specific group/role junior responsibilities vary. Safe bets to mention include preparing pitches (using excel to create graphics and building ppt decks), coordinating information flow (CIMs, managing data rooms, scheduling calls, tracking auction processes, etc), basic financial analysis like spreading comps, and financial modeling, though this tends to mostly be the purview of 2nd years in my experience. It never hurts to be humble and say something along the lines of "I understand that at the junior level I'll be doing a lot of powerpoint but I am really excited to learn about how companies operate financially along the way"). Hopefully this gives you a place to start.