In reality it's more often than not a role-playing exercise where you are pretending to be coworkers trying to solve problems together. Sure you'd be the one leading the problem solving, but being capable to explain your thought process effectively, having the ability to exchange ideas with the interviewer, and just being able to come across as a good teammate is probably more important than getting that last 5% of optimization.
This is especially more true for senior level position interviews where there are more design/architecture problems with relatively open answers. Out of these companies Google felt most impersonal and "test like", which I guess is not a surprise considering they mostly don't hire for any specific teams/positions (and we can debate the pros/cons of that all we want), and they try to eliminate human factors by the way of having hiring committees (which leads to other pros/cons).
In the end even though most still comes down to the technical skills, walking in with the right mentality and attitude actually really helps one to emphasize one's strengths.
Oh one last thing, showing confidence is also super important. But too many people mistake arrogance for confidence. In my personal experience there is no better way to demonstrate confidence than being comfortable (not to be confused with complacency) with what you don't know, and showing intellectual curiosity/eagerness to learn.