Where are stored (geographically) AWS RDS Backup

It is stored in the same AWS region where the RDS instance is located.

When you directly store data in S3, you store it in an S3 container called a bucket (S3 doesn't use the term "instance") in the AWS region you choose, and the data always remains only in that region.

RDS snapshots and backups are not something you store directly -- RDS stores it for you, on your behalf -- so there is no option to select the bucket or region: it is always stored in an S3 bucket in the same AWS region where the RDS instance is located. This can't be modified.

The data from RDS backups and snapshots is not visible to you from the S3 console, because it is not stored in one of your S3 buckets -- it is stored in a bucket owned and controlled by the RDS service within the region.

According to this :

Your Amazon RDS backup storage for each region is composed of the automated backups and manual DB snapshots for that region. Your backup storage is equivalent to the sum of the database storage for all instances in that region

I think that means that it is stored in that region only and s3 stores data like this :

Amazon S3 redundantly stores data in multiple facilities and on multiple devices within each facility. To increase durability, Amazon S3 synchronously stores your data across multiple facilities before confirming that the data has been successfully stored.