Depends on the company you're interviewing with. But here's a description of how companies who insist on culture fit evaluate candidates.

Generally an onsite interview comprises of a series (minimum 2, may go up to 4) of technical interviews which might involve whiteboard coding or coding on a laptop (my current company use this approach - partly inspired by Stripe's interview process where candidates are told to bring their own laptops and use their language/editor/IDE of choice). The coding itself should take anything from 20 (very strong candidates) to 40 minutes, and the remaining 15-20 mins is used to ostensibly allow the interviewee to the interviewer(s) questions, and sometimes these questions/discussions are used to assess culture fit. Like, is the interviewee opinionated about something (which is them used to 'score' the interviewee between 1-10 on 'ego').

Some companies insist on 'lunch' with the candidate, which WOULD be nice if you're having lunch with your team members/engineers, but a lot of times these you're just shown the way to the cafeteria and told to help yourselves, where you find yourself among a sea of strangers laughing/talking about something and you try making small talk (if you're socially awkward or introverted, you fail this test right away). Now, not all companies' lunch interviews are this bad, but some just insist on one and do NOTHING to make the interviewee feel at home (maybe that's the whole point?).

Then there would be a separate 'culture fit' interview AFTER all the coding interviews, usually with the CTO/VP of Engineering/Director of Engineering. This would entail being either asked to rate yourself on a scale of 1-10 on specific tools/technologies, asked about how you deal with conflicts, how you disagree with others when you have to, what side projects you work on and so forth.

At some companies, a culture fit interview is an interview with non-technical folk, who'll ask you for the UMPTEENTH time why you want to work at this company, what are your hobbies, what do you do during your weekends/in your free time, what are your favorite bars, where do you hike/work out, what books you read, what artists you listen to yada yada.

Some companies also insist on the candidate grabbing drinks/breakfast with current employees. I've done a 'breakfast' interview where ALL the engineers were drunk to varying degrees (they'd just had a huge party the previous evening). Generally, this is again a way of seeing if the candidate is someone you'd like to grab drinks with, or would like to hang out with socially after work. Depending on the company, this might involve speaking about tech, or as often as not, not saying a word about the job and just treating it like an evening/morning with old friends.

It's hard to know what exactly to say, and one has to walk the tightrope between sounding and seeming confident and cool and NOT coming across as egoistic/too opinionated and obnoxious. A lot of times, the same answers garner different responses from different teams, and in general, I personally believe these kind of interviews/questions should be banned since it's really no indicator of skill. The people I enjoy hanging out with may not always be the people I want to work with 40 hours a week. Unfortunately, likability is a major factor that goes into the hiring decision.