The PI is usually a PHD with years of experience in charge of a lab, their job is to conduct research and answer questions. They'll bring on post docs and PHD/MSC students to conduct that research, write papers and progress thesis projects. Source: got my MSC in nutrition a few years back then spent a portion of my career in a pathology service core for the local research community so I've seen a lot of different projects over the years (spicifically biology/histopathology).