After years of being compelled to use apple services on iPhone by default, those limitations are finally easing up a bit. As noticed by MacRumors, the ios 14.5 beta appears to let you set third-party music services, this means you can ask Siri to particular music and it will go straight to Spotify or YouTube music.
Currently, Siri is only limited to searches and plays from music on the apple music app. After the ios 14.5 version is installed, Siri will ask you what music services you will like to use and then ask for your permission to access data from your chosen music app after that it will play from that services by default.
When asking Siri to play music it’s better to specify from which services otherwise it’s assumed you want it to play from apple music this will certainly make things easier.