when you create an ec2 snapshot, which s3 is it stored in? is it a random aws managed s3? how secure is it then?
You take snapshot of the EBS Volume attached to the EC2 instance. The location of the snapshot on S3 is abstracted from us. However, we can access the snapshot through the API or the management console but cannot access it via S3.
regarding the security of the snapshot, the snapshot of an encrypted EBS volume is always encrypted.
Refer to Section "Encryption support for snapshots" at https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/EBSSnapshots.html