Hello robertinjo,
Typically Siri will ask what player you want to use the first time a request is made. After that, it continues to use that same player unless asked to use another.
Luckily, Siri is a creature of habit. If you continue to ask Siri to play a track using a specific app, Siri should eventually ask again what app you prefer, or will learn to start using that app.
These tips are found here, Use Siri to play music on iPhone - Apple Support:
• Play Apple Music: You can play any song, artist, album, playlist, or station. Say, for example, “Play ‘Gasoline’ by The Weeknd” or “Play Kehlani.” Siri can also play popular songs from different genres or years. Say “Play the top songs from 1990.” You can also play music on AirPlay 2-enabled devices in a specific room. Say something like “Play the latest album by Camila Cabello in the living room.”
• Let Apple Music be your personal DJ: Say something like “Play my Chill Mix” or “Play something I like.”
• Add music from Apple Music to your library: Say, for example, “Add ‘Evergreen’ by Omar Apollo to my library.” Or while playing something, say “Add this to my library.” (Apple Music subscription required. Not available with Apple Music Voice.)
• Add a song or album to a playlist: While playing a song, say something like “Add this song to my Workout playlist” or “Add this album to my Sunday Morning playlist.”
• Find out more about the current song: Say “What’s playing?”, “Who sings this song?”, or “Who is this song by?”
Tip: You can also identify a song playing on or near iPhone by adding the Music Recognition control to Control Center. Go to Settings 
> Control Center, then tap 
next to Music Recognition. With a song playing, open Control Center, then tap 
.
• Play songs based on their lyrics: Say “Play the song with the lyrics” and then say a key phrase from the lyrics.
Have a good day.
