You're missing the part where these Android OEM fork the OS, add all of their source code modifications, compile, link and the build it. Google cannot update an

You're missing the part where these Android OEM fork the OS, add all of their source code modifications, compile, link and the build it. Google cannot update an OS created by another OEM because unlike Microsoft, which provides these OEM's binaries of their OS, Google simply provides the source to Android.