where does the official, "non-custom firmware" Android os look for version updates? When the "new update is available" popup appears on the screen, does the os go to the manufacturer's server or the google's servers to download the update? in other words who is the main version update publisher that the os looks for updates from?
The OS looks for updates from the manufacturer's site for a newer version. When you click download on your phone it download the latest update usually as "update.zip".