A handful of in-app renders were found by MacRumors contributor Steve Moser. These match up quite closely with photographs of the real deal submitted as part of filings to the FCC.
The Spotify car thing design has a large knob on the right, a color screen, and four buttons on top. Earlier official company renderings from back when the product was formally announced in 2019 showed off a much smaller screen.
Spotify’s design has moved much closer to a smart display style. New renders show off the screen and a possible interface design. Various designs for attaching the device to different parts of the dashboard.
The device was announced in 2019 as just a test product to help Spotify better understand users’ listening habits while on the go.