uevent sent from kernel to user space (udev)

I knew that udev plays on the linux system and it receives uevents sent from the kernel through netlink socket. However, my questions are: How kernel sends out the event ? It must be something triggered by adding/removing device and then sends out events to udev. How does kernel do this? (Is there any code example I can find? ) udev receives these uevents only through netlink socket. This is the o