Creating Custom Background Voice Intent for Android App

Question

I know this has been a topic covered for 8 years, but I haven't been able to find a recent answer to this question.

I'm working on an Android Auto app in my spare time. I'm trying to build out a version of the License Plate Game; when you spot an out-of-state license plate you log it to the app. And hopefully you can find all 50.

Speaking with my wife last night, we realized the smartest way to achieve this would be to activate speaking from the car's steering wheel. From there, an utterance of something like Add Utah to the License Plate Game would add Utah to your list of license plates you've seen. A user clearly wouldn't want this app in the foreground for the duration of a car ride, so a background voice action would be the way to go.

The Android Auto portion would just be for fun and to learn how to develop for Android Auto with some kind of minimal UI.

Documentation on Voice Actions for Android is minimal when it comes to custom voice utterances. Would I really need to create a Google Assistant extension for an app that is intended to store data and have all logic hosted locally within the project?

Thanks for any and all help!