HIkvision (and similar) guides are a little clunky when trying to understand how it all comes together but it is essentially a system where the NVR is central to the wider system. Whenever a change is made on the NVR, it sends that config to the camera in question which does the grunt work. It is meant to be centrally administrative. So if you want a line crossing event, you configure it as a Smart Event on the NVR for that camera, add the schedule it should apply and the triggers. Altogether, any event that is detected will then be fed back to the NVR which logs both the video and the time/event VCA data so that you can easily see it on the timeline (be sure to tick either the Human or Vehicle tickboxes under it). Notifications are configured the same way.
Yes, depending on the NVR, most pro installs use POE switches for the cameras and you simply add the IP addresses to the NVR for it to do its thing.
Good luck.