What does "slide to upgrade" do?

On iOS 9, after restoring from a backup, a screen comes up that says "slide to upgrade."

This doesn't update your iOS firmware, right? I imagine it just prepares your backup for iOS 9. Is this correct? I have scoured Apple's support articles, Reddit, and the likes. All I can find is information about a bug related to the slide to upgrade screen.

I just bought an iPhone SE. It came pre-installed with iOS 9.3.2. I set it up from an iPhone 5s iOS 8.1 iTunes backup. When the restore was complete, I was greeted with the "slide to upgrade" message. I slid it, and a loading bar with the Apple logo was displayed. When it was finished loading, a message appeared saying "upgrade complete." When the setup was finished, I went into Settings; it was still on iOS 9.3.2. It is now asking me to update to iOS 9.3.3.

Thus, "slide to upgrade" does not update your iOS firmware to the latest version, it likely just prepares the old backup data for use with with iOS 9.

Great question! For anyone worried about this I want to confirm that "Slide to upgrade" does NOT update iOS (Apple really should change that message to avoid misunderstandings).

My iPhone X was on 12.4.1, and I just restored from a backup from iTunes and got the same "Slide to upgrade" message. At the time of restore 12.4.1 wasn't signed (only iOS 13.1.2 and 13.1.3 was).

And after doing "Slide to upgrade" my device was still on iOS 12.4.1.