Session restoration works by running your jobs within long-lived servers rather than as child processes of iTerm2. If iTerm2 crashes or upgrades, the servers keep going. When iTerm2 restarts, it searches for running servers and connects to them. The OS's window restoration feature preserves the content of your window, including scrollback history. iTerm2 marries the restored session to the appropriate server so you can pick up where you were.
tl;dr watch this: Demo Video
You must turn off System Prefs > General > Close windows when quitting an app. For more information on system window restoration, please see Apple's docs.
You must set Prefs > General > Startup to Use system window restoration settings.
Force quitting iTerm2, causing it to crash, or upgrading it when prompted should restore your sessions. NOTE: Quitting iTerm2 with Cmd-Q will terminate your jobs and they won't be restored. There is an advanced preference to change this behavior, though.
You can toggle this feature with Prefs>Advanced>Enable session restoration., but you must restart iTerm2 after changing this setting.
A session that has had only its window contents restored and not the running processes will get a reverse video Session Restored banner. A properly restored session will pick up right where you left it.
If you reboot, your jobs will terminate and not be restored. The window contents should be restored.