Although I only have experience (so far) with interviewing for master's programs, I hosted a grad student each year that I was in my MA program. Going into it, I tried to be a good host and offer wifi passwords, laptop use, TV, etc., and just generally tried to make them feel welcome, but also tried to give them space to do what they wanted (ask questions, relax, or be alone). That was pretty much my experience when interviewing, too.
The feedback I gave to the professors after hosting an applicant was just a general impression, similar to feedback after the social event or interviews. If I really liked the applicant, or thought they'd fit in well with our lab (if their POI was my mentor), I made sure to let professors know. I tried to be understanding about negative impressions, since we all say goofy things when we're nervous, but I would have mentioned real red flags like rudeness, a lot of social awkwardness (not just a foot-in-mouth situation), and excessive cursing or badmouthing professors or other applicants, etc. As long as they were relatively pleasant to be around, I didn't care whether the applicant wanted to watch TV, ask questions, or just decompress in their room-- I didn't assume they were a jerk if they just wanted to be alone and get a good night's sleep 🙂
I know that my experience hosting applicants for a master's program may not be very similar to applicants interviewing for PhD, but I hope this information is helpful!