Carplay and Android Auto allow for displaying on the car’s display the GUI some of the applications running on the phone - as well as interacting with them .
Ex, run a navigation app on the phone, have its GUI on the car’s headunit display, interact with by using car’s features (touch screen, steering wheel buttons, etc) and integrate with car audio (ie: mix voice instructions over car onboard audio etc). Its more comfortable and secure than a phone hanging there (IMO).
I’d like SFOS apps, like PureMaps, displayed on my car’s screen 
I think Seafarix intervene more on the headunit side. Still their expertise on the matter should help steering such an effort for SFOS phones (even though that’s a NDA over NDA world )
CarPlay seems more locked down than AA.
See this post from Vige for some AA details:
Sailfish phone usage in cars (like Android Auto or Car Play) - #5 by vige