>>ycombi+(OP)
The source for AOSP. Individual Android devices have never been open source (minus some very few exceptions), and they have no obligation to do so as AOSP is Apache/permissively licensed.
Note that the demand for secrecy is primarily driven by device manufacturers, not Google. Manufacturers want to keep their "secret sauce" from their competitors.
>>Ferret+Ww
Google has gradually been moving things from AOSP (open) to GApps (closed). Some of the things that have been moved are fairly essential for a mobile operating system (like a location provider and a SMS app). Projects building on AOSP now have to provide replacements, or declare them out of scope and punt.