Imagine anyone points their phone at you in the street, and there they are looking at your phone, your contacts, your apps, everything.
You need your iCloud login/password and also (I think) an existing device which has the key to approve the request.
This is literally no different than how it works exactly today when restoring an iCloud backup onto a new phone, except imagine it can happen as a polyfill so it looks instantaneous instead of the 2-4 hours it seems to take today.
polyfill definitely doesn't mean what you think it does.
The Secure Element would need to be upgraded to support "multi-tenancy".
That's all assuming the feature works by building on the existing infrastructure.
Ideally between your face and your iCloud password you could bring your "profile" to new hardware without having to touch an existing device. After the first time, just your face is enough.
Imagine you walk into an Apple Store and pick up the latest iPhone XV. And there you are looking at your phone, your contacts, your apps, everything.
Underneath, it's a virtual shim. In the first instant it's merely grabbing thumbnails of all your apps and notification metadata so it can "look right". As you click in, scroll around, you polyfill data as you need it. Obviously in some cases a more substantial download would be needed, so you may not be able to pop into an AR game with 1GB of assets within the first 10 seconds of picking up the device, but if your primary is on the LAN you could bring the necessary data locally in ~5 seconds from tapping the icon. Underneath it's doing something analogous to "docker run" on that apps image. In some cases this would lock the image from running concurrently on another device, in other cases multi-master could be fully supported with live sync of the backing stream, e.g. for Apple Mail.