The audio input into Google Assistant depends on the vehicle's Bluetooth system. Some car audio systems don't have a built-in microphone. In those systems, the Google Assistant takes your voice commands from the Phone’s microphone.
For car systems that have a built-in microphone, you can enable an option on your Android phone to receive the input from the vehicle's microphone and get a better Google Assistant experience.
You can enable the Bluetooth Audio Recording option as shown below:
Tap Profile and then tap Google Assistant Setup.
Tap Bluetooth Audio Recording.
Enable Bluetooth audio recording.