Two things.
The google interview process was not made to filter you out. It was made to guarantee that they will not make a mistake on hiring you. People with the capability to do those interview problems have a high chance of being good programmers. There are many people like you who can't do those problems but are still very good programmers. Google doesn't need to hire you, the whole interview process is optimized for preventing a bad hire rather than finding people like you who are good hires but can't interview.
Second, traversing a tree in level order is a trivial problem from the interview question perspective. It is more than likely that this question will NOT be given by the interviewer because it is too easy. The interview questions FAANG tends to give are a bit more novel. In fact the method in which you use to solve this problem and the problem itself is often just taught directly in school depending on the class you're in, meaning you don't need to practice interview questions to solve it because it's a basic topic.
The solution is called tree traversal using BFS. If you didn't know this, despite working on and creating things like etcd you will appear stupid because the question is just too basic. When I interview I am 100% expecting questions way harder than this... even on phone interviews. Additionally when I'm the interviewer this type of question is the lowest bar, I will fail someone who can't answer this question.
This is not to comment on your abilities. Rather it is to give you a dose of truth. I truly believe that there are many people like you who can program an entire kernel but can't even solve fibonacci recursively on a white board. Despite this I can't stop judging you based off of the white board question nor do I know how to confirm that you can write a kernel despite lack of ability to solve very very basic programming questions.