Senior Scala interviews focus on depth of understanding rather than breadth of knowledge. Interviewers are less interested in whether a candidate can list language features and more interested in whether they can explain how those features interact.
Companies hiring developers with several years of experience expect candidates to demonstrate practical reasoning about functional programming patterns, concurrency models, and the Scala type system. A mid-level engineer is usually expected to work independently, contribute to design decisions, and understand the trade-offs behind the tools they use.