iOS 14 lets you set a default mail client and default browser, but Apple clarified today with TechCrunch that it definitely does not allow you to set a default music player.
The confusion arose after a new feature showed up in Siri for iOS 14.5 Beta 1 called Audio Choice, where Siri would ask you which music player to use when you ask it to play a song. After you choose a service Siri would then subsequently use the same music service e.g. Spotify to play future tunes.
Apple today clarified that this was not a fixed setting and that it could change in the future and Siri could even ask you to choose again. It may even change your preference if it notices you use another service consistently after or use a different service for different things, e.g. Spotify for podcasts and Apple Podcasts for podcasts.
Apple says learning your preferences is Siri becoming more intelligent, but it remains to be seen what the European Union and Spotify think about such an ephemeral ‘user choice setting‘.