How open source Android really is? Would you be able to compile a decent phone entirely from source like you can for a desktop like Ubuntu?
Almost all Google apps are closed source: see this picture: http://arstechnica.com/gadgets/2014/07/exploring-the-world-of-foss-android-can-a-smartphone-be-open-source/ Those apps are called Gapps (Google Apps) by some devs.
Development is closed, and Google just releases source after the fact. This is called by some a code drop, since it looks like an airplane resource drop in some games (and real life as well I suppose).
I don't think it is possible to simply compile and install the source given by Google (TODO what is missing?), thus things like https://wiki.cyanogenmod.org/w/Development exist.