According to some Reddit users, asking Siri to play a song in the iOS 14.5 beta brings up a menu that lets you choose a default music provider, although it seems that only some are supported.
One of the issues with Apple’s mobile ecosystem has always been the lack of an ability to set default applications. Just this year with iOS 14, users finally got the ability to change their default browser and email app. As spotted by some savvy Reddit users, Apple might be expanding that to include the default music app. The idea is that if you ask Siri to play a song, it will automatically do so in the service of your choosing. As it stands right now, if you want Siri to play a song in Spotify, you have to specify that the song must be played in Spotify.