Hi @John_Spaventa,
This ultimately comes down to the fact that ARC doesn’t have fully integrated Google Assistant integration. YouTube Music and other apps are natively integrated with Google Assistant for Android Auto; from the head unit’s perspective, there’s no way to flag ARC as the default app for media playback under the current implementation.
The team is aware of this limitation and actively working to expand ARC’s compatibility with Google Assistant.
I recommend creating a post in Feedback > Feature Suggestions that outlines your desired interaction with ARC through Google Assistant. Our senior development and product teams will certainly take this feedback into account since this work is ongoing.
Thank you again for your post.