It's in TFA: "the Pixel 4 also controls the motors and other electronic components via USB-C".

There's an STM32 micro-controller which presumably acts as an USB slave and controls the servos.

what SDK is this ? i have hunted and not found any easy way to do serial I/O over usb-C in the android SDK

Just google "arduino android" the interface had been used by many projects for so many years.

Google "USB motor controller"

It doesn't quite say in the article but I'd guess that they're using USB connection with accessory protocol based on:

- Their chosen controller doesn't have Bluetooth.

- The images show phone cradle having cable connection to the phone.

- There's an off remark in the motor segment that they made the external board power the phone implying they were connected before.

- It's the easiest way and supported by Android APIs.

Android Things as a project is dead and it never ran on phones - it was always meant to power IoT devices with screen.