Expert questions test whether you can combine patterns under constraints: a rolling count of distinct users (windows can't do DISTINCT), a median without a percentile function, an as-of join with open-ended ranges, deduplication with a tie-break rule. Interviewers also ask you to reason about plans and trade-offs.
Talk through the grain, the edge cases and the cost of your approach. All questions are written for this site and use Kestrel's tables.