From a quick search, this is what I found about how to set your default music app on iPhone and iPad
- Ensure your iPhone or iPad is running iOS 14.5 or iPadOS 14.5
- Ask Siri to play "music" or request a specific song, album, or artist
- Siri will ask where you'd like to play that music from and a list will appear on-screen
- Choose from any of the supported audio apps you have installed, including Spotify
- Your audio should start to play from that app and each subsequent request will default to that app
Note: Currently, there is no way to change this again. It is not listed as a setting, and you can't ask Siri to update your "default" player. If you want to play from a different source in the future, be sure to explicitly state in your request which app you'd like to use.