What OS (Android) Do We Use For own Product And its Hardware?

Keywords´╝Ü android linux operating-system hardware-programming

Question: 

I have some experience about internet of things and MCU programming. I wanted to know to build a product like mirror or tonal or something like them. Android TV or Car monitor. What operating system is used? I guess all of them using kind of android/Linux but how can they develop it for own hardware ? Recently I have read about Android of things and I understand that it supports a number of specific hardware like Raspberry Pi 3. If I want to create a product like these products, what should I know about the developing operating system and its prerequisites?

It is a good idea to introduce a resource or book about all process. Thanks a lot

Answers: