Hi nsp84,
welcome to this Apple (user-to-user) Community!
Re: "Making my downloaded music default : i bought a new 17 pro. I live in a rural area with limits internet. When I get in my truck and it connects to my phone the blue tooth music streams from Apple Music instead of playing my downloaded music. Because it’s always trying to stream I loose my music".
Am no expert in this area, yet your request deserves some response:
We don't know if you are a Music subscriber, or if you mostly use the Music app for files from CD's. etc
Here a few suggestions below, and ai least we get your discussion underway.
That provides opportunity for others with more expertise to share their ideas and experience.
Seems AirPlane mode is good for disabling wi-fi and cellular altogether.
or
In the Music App > Library > There's a section called "Downloaded"
See if you can choose to just play music from there,
or
If you sync music to iCloud, turning off sync, may let the offline music play
(Settings > Music > toggle off :Sync Music")
or
In Settings > Music > Downloads > Audio Quality
Seems you can configure the music quality just for your downloaded music.
or
There are also third-party (non-Apple) music players (like VLC app)
They can also play downloaded music files
All the best :-)