I don't think they document how how they're doing things in the background. Based on personal experience I've been notified that EBS-backed instances have been rebooted due to hardware failure. For instances with ephemeral storage you're given some time to migrate for scheduled maintenance, for hardware failures you're tough out of luck. I've never been notified about scheduled maintenance on an EBS-backed instance, which leads me to think that they just migrate it without telling you.

They're using KVM under the hood, which supports things like live migration, interrupts to facilitate memory transfer, and so on. At home I'm doing live migration with less than 300ms of interrupts with high memory pressure. It would make sense for them to move things around, just for things like maintenance and distributing load.