Unlike Bluetooth headphones optimized for Google Assistant, optimizing USB-C headphones for the Assistant does not require integrating Google software into the device itself. This is because the Assistant functionality sits within the Assistant app on the user's phone. As long as you create headphones that are compatible with the device requirements, your headphones will be compatible with the Assistant optimizations in the Assistant app.
You may freely build and test the Assistant optimizations for your USB-C headphones device using the information shared here. However, to enable this functionality for end users of the device, you will need to make the necessary business arrangements with Google and submit your device for certification.
A light-weight legal agreement is required to launch a device with Assistant integration. Reach out to your Google business contact for more details on this agreement. If you don't have a Google business contact, you can send an email to firstname.lastname@example.org.
Once the necessary business agreements are in place, you will be given access to additional documentation that describes the commercialization and certification process.
You will need to provide Google with basic device information used to customize the details shown to the user when enabling Assistant integration on the phone.
Every Assistant device must be certified to validate performance and functionality.
After you complete these requirements, Google will whitelist your device and turn on the Assistant optimizations for all production users.