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.
My team does whiteboard questions, but we try to keep them practical.
Typically they are the types of problems that we'd expect new engineers to have to look into on their first day. Oftentimes the questions are less "come up and an answer" and more "let's explore this problem domain and see what we can uncover."
As an example, the interviewer will write up on the board a data structure that is too big and has redundant fields in it, and start a discussion about how to optimize that structure. The optimizations go more and more in-depth as the candidate gains experience thinking within the problem space.
Another problem, the interviewer will give an example of a synchronous operation and work with the candidate to break it up into a set of asynchronous calls.
My team tries to avoid "puzzle questions with 1 correct answer".
I do ask simple data structure questions just to test basic programming skills. On occasion someone does make it through who cannot even write a FOR loop.
My standard go to question is "Please do an in order print of this already sorted binary tree."
It is 5 lines of code, and ~80% of candidates who haven't seen a tree in a decade can still figure it out.
IMH the goal isn't to try and trick people, it is to try and see if people can hold a conversation about a technical topic and make valuable contributions to a discussion.