How it actually works
Every medication, vitals entry and daily note is stored in your own private iCloud account, the same one that already backs up your photos and messages. When you open Halo on another of your devices signed into that same iCloud account, the records are simply already there.
Why this matters more than it sounds like it should
For a single-device household this is invisible. For anything else, a phone and an iPad, a shared family device, a phone upgrade, it's the difference between a continuous record and starting over. Nobody wants to rebuild six months of weight history because they switched phones.
What it needs to work
Just an iCloud account signed in on each device, and iCloud enabled for the app in your device settings, which is on by default. There's no account to create inside Halo itself and nothing extra to configure.
What happens if you're offline
Everything still works exactly the same, logging a dose or checking a reading doesn't need a connection. It syncs the next time the device has one, quietly, in the background.
This is the same principle behind why Halo doesn't collect your data in the first place: the records were always meant to be yours, sync just means "yours, everywhere," not "yours, on a server somewhere."