Android OS is typically pre-installed in various Android devices, which includes smartphones, tablets, cameras, smartwatches, consoles, and so on. Updates to the OS happen periodically, and you can check on your own by going under Settings>About Phone>System Updates.
Developers have served up all versions of Android on the web, but to install them on a device (especially a non-Android device), it has to be one that has development out there to port the OS, and you'd have to be comfortable enough to do this on your own.
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Sorry, Android OS was embedded in the phone itself. Althrough you can flash another ROM of the Android, you can't ( it's almost impossible ) to install Android in an un-native android phone.
Yes, you can download the source code of Android by heading over to the developers section of Android.
Android OS uses applications that are a customized version of Java. Without knowing the reasons and where this is being downloaded to, it is difficult to answer this question.