I am using sys-usb for more security. what is really happen while attaching usb drive?
What if my usb was having virus ? is it could compromise my templates or even dom0 ?
What is the most secure way to attach usb drive ?
Start with this:
And if you like to get into the details, read this:
Apologies for sounding abrupt, and I love the fact that you want to understand more how Qubes OS actually works, but each question you’re asking results in more six more questions being asked, and most of those questions are mainly trying to clarify your initial questions 
I am using a computer to be more technological… 
That will be explained in the qrexec docs. Sadly it isn’t really something that can be condensed into a single sentence or paragraph. You kind of need to read the whole thing 
Well, depending on the kind of malware your USB drive had, and what system components it targeted, different things would happen… 
Why yes, depending on what it targeted… 
Plug your USB drive into the USB port and weld it to your computer. Nobody will be able to mess with it afterwards 
But seriously, I have a feeling most of your questions will be answered by having a read of those docs I sent you 
I disagree. When you attach a USB drive on Qubes, it’s connected to a virtual machine and isolated from the rest of the system. This protects you from many USB attacks. In a typical case (when sys-usb is a disposable VM), a reboot of sys-usb will reset it and your sys-usb will be secure again. In the worst case, your sys-usb controller may become compromised and might compromise everything you connect to it later. See also: Should we have one "sys-usb" per USB Controller? and Proposed procedure for using untrusted USB drives.
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