Probably9857 more specifically
Your device scans the update list every 4 hours
Once an update is pushed to 'stable' (meaning it pasts user bug testing), your device sees it and downloads it
You will receive a notification requesting you to reboot, simply press the button and wait for it to reboot
It will be mostly seemless once you reboot, tho depending on how many apps you have installed and what was updated, the phone may have to recompile apps which takes some time. You can still use the phone while it's doing that it just may be a bit slow. A status bar shows the progress.