Android is more of an open core project. AOSP is technically fully open source but when people say Android they mean the Android version of Google with the play store and play services.

The naming is confusing because what people call Android is not actually Android like what people call Linux is not actually Linux but a distribution.

Even with ASOP it tends to be more of a code dump than an open source development process. If you're an OEM you get to see it earlier in development (and get to dump your own OEM patches in there) but for most people the actual development is done behind closed doors.