Do we know how Apple sends these? Is it just a notification, or also email?

https://support.apple.com/en-us/102174

>A Threat Notification is displayed at the top of the page after the user signs into account.apple.com.

>Apple sends an email and iMessage notification to the email addresses and phone numbers associated with the user’s Apple Account.

You can see what it looks like in https://reddit.com/r/iphone/comments/1c10jai/i_have_received...

I wonder how they detect it, is it for known IOCs that they've already found elsewhere, or do they have heuristic detection that flags things that might need further investigation.