The process for designing apps in Android for Cars depends on the type of app and its use case: whether it will be used while driving or while parked.
Some apps are designed using a set of templates spanning both driving and parked use cases. Others, such as media apps, involve fitting your app content and actions into a driving-optimized UI. Finally, for parked and passenger experiences in cars with Google built-in, you can simply adapt an existing app for the car screen.
As you design your app, keep in mind that how users experience your app will depend partly on whether they use the Android Auto version or the AAOS version (which is customizable by car makers). For more on who contributes what to the design, see Partner roles.