My experience with Apple Carplay and Android Auto is compatibility for both is built in a number of brands as well as aftermarket devices already.
Brands like Hyundai already offer both on their cars in addition to their own interface and app. Shouldn’t be impossible, therefore. BMW afaik can integrate all 3 (Apple/Google/Amazon) in newer models and provides choice to the user.
Apple Carplay/Android Auto are ahead in integration with car and appliance brands offering an integrated driving experience and most of the time provide a user choice. Alexa is catching up fast. Don’t know if Amazon goes for hard integration/factory installation with brands more instead of allowing consumers to choose based on preference as it will be independent of the phone platform choice, which Apple and Google have to bet on.