Where does macOS Get Icons for W1 Audio Devices?
I was able to find where icons are stored for W1 devices. It appears that this folder already contains assets for every W1 device that exists. You can find icons for W1 devices here:

/System/Library/Frameworks/IOBluetoothUI.framework/Versions/A/Resources


If you're using Swift on macOS, you can use the following code to programmatically get a device image:

IOBluetoothDevice.init(addressString: "bluetoothDeviceAddress").value(forKey: "image")


or

   for device in IOBluetoothDevice.pairedDevices() {
    let deviceName = ((device as! IOBluetoothDevice).name)
    let deviceImage = (device as! IOBluetoothDevice).value(forKey: "image")
}