AWS Backup, backup vault underlying storage

I would like to inform you the underlying storage for Backup Vault is indeed S3 service.
Backup service basically stores data in the internal AWS S3 buckets. Being internal, these S3 buckets are not accessible to you directly and AWS manages the same in the backend.

AWS Backup has 2 storage options, Warm and Cold storage. Cold storage option basically provides a low cost storage option, then the warm storage. Major pointers to note about cold storage being:

a. If you want your backups to be incremental, you must have at least one recovery point in warm tier.

b. If you set your lifecycle to back up to cold storage after 1 day, each of those backups will be a full backup. Therefore, AWS Backup recommends that you set your lifecycle settings to not move your backup to cold storage until after at least 8 days.

[+] Creating a backup plan: https://docs.aws.amazon.com/aws-backup/latest/devguide/creating-a-backup-plan.html#plan-options-and-configuration