How to handle 'design X from scratch' when you've never used X?

I recently had an interview where I was asked to 'design X from scratch,' and X was a domain I had very little direct experience with. Specifically, it was designing something similar to Spotify, and while I understand general distributed systems, I haven't worked with audio streaming internals at scale. I felt pretty out of my depth. How do you approach these types of questions without direct prior experience? My initial thought was to break it down into familiar components (user management, content storage, search, recommendations) and then acknowledge the unknown parts (e.g., 'for audio streaming, I'd assume we need efficient CDN delivery, possibly adaptive bitrate streaming, but I'd need to research specific protocols'). Is it okay to admit ignorance on specific low-level details as long as you can reason about the high-level architecture and identify areas for further investigation? I'm looking for a solid framework to reason about unfamiliar domains in an interview setting, demonstrating a generalist system design aptitude rather than specific domain expertise.