When the device does its activation the activation server see that it is attached to DEP, so it sends it off to DEP, which then sends it to the server it was assigned to, which then sends the relevant data to the iPad so it can run through the enrollment process as determined by the prestage it's assigned to.
For iPads, the activation always happens after connecting to wireless, and only happens once. This means that if you start the setup before putting the device in a prestage's scope you'll need to wipe the device to force it to activate again. For OSX activation time varies depending on the method used to install the OS. We've recently discovered that with our 10.13+ netinstall we must have the device assigned to a prestage before the install completes for it to enroll, as the activation occurs immediately on first boot after the install.
