How exactly does operating system update work on Android devices? Today I have received my first Android device, to be more precise a GOClever Tab A102.2 tablet. I have noticed that it runs as operating system Android 4.0.4 so I though why not update it since the latest one is 4.2? I went to Settings > About > System Update however it asks for a update.zip file which I have to place in NAND or SD card.

Searching through the internet I have found that some devices connect to the internet and check for updates when prompted while others need to be updated with vendor specific versions (I guess that's the case for me).

Now I wonder, why do some devices need to be updated with vendor specific versions? The way I see it, Android is just an Operating System and thus not tied to the device so theoretically if I wanted I could ditch it in favor of some other operating system (say a linux distro) so whether the update comes from the vendor or not should be irrelevant. 

Also, can vendor specific versions of Android be updated (through some other means) even though the vendor didn't release an update for it?
Most vendors/manufacturers modify the Android source code to ensure full compatibility with the devices hardware - if you were to update to a clean version of Android, it just wouldn't work properly (and it it even booted, most things wouldn't work at all).

Some manufacturers implement an OTA update feature - whereas some don't. Those that don't sometimes make you connect your phone/tablet to the computer and use their 'helper' program to do the update.

Even the devices that do connect to the internet and download the update are vendor specific - device specific even, it is just that the manufacturer let you do it that way.

As to your final question: Yes, through a custom ROM. Custom ROM's are versions of Android that have been modified by the Android community, based on the stock Android code, to work on your device. You should search google, or look at forums such as XDA-Developers for more info about ROM's, or to look for a ROM for your device.

Remember: All the hardware in phones are different. Only the manufacturer knows how to code for it, so that is why default Android code won't work with it - it would need propriety drivers.