The Android Open Source Project is essentially the Android source code for Nexus devices and the Pixel C. However, it's somewhat different from the internal source tree used to build stock Android for those devices.
It's important to note that it's only closely tied to the internal source code for stable releases. The master branch of AOSP does not closely track the internal development. The internal work is released as part of a stable update, and then merged into the AOSP master branch afterwards. AOSP master and the internal source tree are repeatedly merged into each other to some extent during a year of development, as some work happens out in the open (lots of the low-level userspace work) while other work happens internally. For some repositories, there's substantial amounts of both. The merges are primarily from AOSP into the internal tree until new stable source code is released. These strange workflows result