What does the "Transfer Using Quick Start" do?
Apple offers several methods to transfer settings from an old phone to a new one. A particularly promising sounding one is the "Quick Start" approach, which begins promisingly enough with some cool automatic recognition of the new device. But things pretty much end there. Nothing about actually transferring seems to be any different than any other method: one still has to restore from either an iTunes or and iCloud backup as far as I can tell.
What does the "Quick Start" method actually transfer? Is restoring from a separate backup still necessary to complete the process of transferring settings, apps, and keychain info from the old iOS device?
Despite the description on Apple's website, Quick Start is not a distinct method for transferring content from an old to a new iOS device. It's simply a way of skipping a couple of the more laborious screens in the setup assistant on the new device. It is still necessary to perform the usual steps to ensure that the backup from which you choose to restore is fully up to date. (In particular, the instructions to first unpair any Watch because this "automatically backs up" the Watch, are incomplete at best.)
Using Quick Start, you will skip the following setup assistant screens on the new device:
- Choosing a Wi-Fi network
- Entering your Wi-Fi network password
- Entering your Apple ID for iCloud
- Entering your Apple ID for iTunes and App Stores, if you use a different one
Using Quick Start, you will need to complete the following additional screen on the new device:
- Entering your old device passcode (which you may have to do anyway depending on your setup, regardless of whether you use Quick Start or not)
Quick Start has no effect on how data is restored. You must still have first created an up to date backup, and select that as the source from which to restore.
The guide you link to - step 6 explains how "Quick Start" automates away many of the taps to set up the new device using an iCloud backup. This does rely on bluetooth and wifi to enable a continuity connection to share the status of a backup and initiate the backups / restores.
Your new device offers the choice of restoring apps, data, and settings from your most recent iCloud backup, or updating your current device's backup and then restoring
Wifi is still required to back up to iCloud and download the larger apps once the iCloud data and list of apps needed is downloaded from iCloud. Continuity ad hoc network is just used to signal the status and share wifi credentials, not transfer all the data.
You didn't ask, but in case you wonder how to speed things up, we set up caching servers which lets your iCloud backup be stored locally as well as caching app instals. This speeds the backup and setup process sometimes 4 to 8 times as fast as if you had a very good internet connection and no caching server. If you have a caching server at home and backup for several nights, you might see 10x or even higher speedups based on your internet speed and the cache being fully primed with the data and apps you need.