Brief introduction and tour of facilities It was clear I had been selected for the position before the interview happened, so the interview was more of an introduction to my boss and the lab I was working in

Your interview is with the specific research mentor you have been matched with (if a match is made after submitting your application). Therefore, the interview entirely depends on your mentor; it's not at all standardized. I just met my mentor on campus, and she asked me very informally about my interests and told me about her research. She essentially told me just to let her know if I was interested and that was all.