Apps like Xender, allow a user to share an app with another user, by directly transferring (and installing the app) on another users device. This is done device to device, and done without the target device downloading the app from google play.
I already know how to transfer a file from one device to another using wifi direct.
My question is - what file do you need to transfer, to transfer the app? Is it the apk ? Where is it located ?