The ability to search in Roon ARC is missing from the Android Auto implementation. This is available in every other music player I have used (Spotify, Qobuz, Symphonium, Plexamp) on Android Auto. The implementation is generally a floating search button. If the vehicle is moving, search is restricted to voice input only. Otherwise, a keyboard is available.
I do not see a current Feature Suggestion for this. I have found other forum posts about it, and the one workaround mentioned is to use Google Assistant to “Play X using Roon ARC”. I tested this briefly and was unable to get it to work before my Android Auto was “upgraded” to Gemini. Gemini happily complies with my request (responding with a “Playing Dark Side of the Moon using Roon ARC”), but nothing actually happens.
Here’s a recording of how Qobuz implements this, obviously parked and not driving. 