I dont use Carplay but I do use Spotify on Android Auto and the only way I can search for music is by voice command. I presume Carplay implements Siri to some degree. I just ask "Play the song Alright by Kendrick Lamar" and it will start playing instantly. The lack of a search function is a safety feature to stop people driving and typing on a screen. It's a dangerous distraction to be staring at the screen so it make sense.