Videos interrupt music on HomePod

I bought a HomePod mini to listen to classical music. That’s all I want to do on it - don’t need it to do anything else. How can I make it just work for the Apple Music and Classical apps? If I’m browsing FB or the news or anything else with video on my iPhone it interrupts the music on my HomePod and starts playing there. I’ve checked all the settings and can’t find anything. I turned off all the AirPlay handoff settings and it still does it. I want the HomePod to only ever be used for playing music from Apple Music or Classical. I bought it to replace an Amazon Echo thinking it would be better, but this is actually worse.