How the device (that is off) decide how to publish itself for USB: "MediaTek PreLoader USB VCOM port" or "MediaTek USB Port" device?
I can't see the question here ? 

Are you asking what the driver is for ?   

Please have a look at this video to understand what the preloader is, MTK Backup &amp; Restore  

The Mediatek Preloader is used for flashing new image's to your device.  

Their is two different boot loaders on Mediatek device's.. 


MTK DA USB - For flashing the Preloader   
MTK Preloader  - For flashing all other image's     


How does it register as a device while it's off ?
This is so you can fix your device when it won't even turn on, it need's to be before the boot of your system.  

I hope i somehow answered the question here.