Earlier quoted context omitted.
How do you practice answering whiteboard questions?
By re-learning computer science concepts you haven't had to use in real life without Googling in a decade.
Generally, you have a problem, you research, you experiment and then you wrap all that in a nice easy to use utility class or api and forget the details because there is something else you need to work on and that problem was just one piece of the puzzle anyway.
The worst hypocrisy in all that, is if you ask a specific question to a veteran developer on a average size codebase, he will very often need to look in the code and you will not think any less of him. But if it is an interview and he does not remember some trivia in a piece of code he didn't write - my god, he is a loser.