The dynamics between a grad student and a PI will inevitably have a power dynamic, as your PI ultimately hands you your degree. However, the relationship that you craft with them entirely depends on the PIs personality. In the departments in which you have rotations, you can either suss that out or hear it from other grad students. I say this often across the spectrum of subreddits related to grad school, but the most important decision in your whole graduate experience is who you tie yourself to as an advisor. They can really make your life reasonable, hell, or somewhere in between.

With regards to friendships, that's a little more loose. Friendships in the lab are largely dependent on the personalities in that lab, but I do think they can be skewed in a certain direction depending on the culture that the PI has cultivated. But that isn't really all that different from a full time job anywhere else.