Do phone manufacturers use AOSP as their base? Do phone manufacturers like Samsung use AOSP as their base and customize that, or does Google provide a special vanilla version of Android and they customize that?
I worked with a couple of OEMs and know for a fact that when the OEMs start off working on a new version of android the Software Configuration team picks the latest stable version of the AOSP and then various teams start work to put their customizations in place.
So say a Samsung will have the Touchwiz team start work to customize the UX. The Baseband team will route the Radio Interface layer to the proprietary RIL or vendor-Ril as it is sometimes called. It takes months of effort to roll out phones with the latest software. And contect with google minimal. Google does a compatibilty test suite (for new handset launches) upon failure of which Google proactively supports. Google also works very proactively with OEMs on the Nexus line of phones.