How does Safari get updated on OS X?
In OS X, Safari is updated via the normal system bundled software update process. On Lion, this is the Apple Menu, Software Update tool which can be scheduled or disabled.

On Mountain Lion, this function remains in the Apple Menu, but runs through the Mac App Store app

http://www.apple.com/softwareupdate/

In the end, both tools call softwareupdate binary that you can also run from the command line. 

This differs from your Windows experience where the Apple Software Update tool runs to update all Apple software (Safari, QuickTime, iTunes, etc.) since it is not part of Microsoft's software update process.