How does antivirus find the files accessed by an application without root access? I haven't rooted my mobile phone, but an antivirus is running. Without rooting, how will the antivirus come to know about the files accessed by an application?

Normally, in Linux distributions, it can be found from 
/proc/"p-id"/fd, but in Android root privileges are required to access that folder, although it is not the case in normal Linux distributions. So, without rooting, how does the antivirus come to know about the files accessed by an application? Is there any other way other than accessing /proc/"p-id"/fd?
It's simple - It doesn't. Antivirus on android has very limited access. It can't check what the given app is doing. It's not very useful. It may warn you if it finds something suspicious on sdcard/externalsdcard or one of your installed apps is in their database marked as bad. It can't scan the app data files, it can't scan the main system, so it's pretty pointless to use it.