This should shade light on your concern:
Well Amazon S3 bucket names are globally unique, this is regardless of the AWS Region in which you create the bucket. You are expected to specify the name at the time you create the bucket.
Please Note Amazon S3 creates buckets in a region you specify. You have the freedom choose any AWS Region that is geographically close to you. This helps in optimizing latency. It also minimizes costs or address regulatory requirements.
Objects belonging to a bucket that you create in a specific AWS Region never leave that region unless you explicitly transfer them to another region. That is why the user interface shows all your buckets, in all regions. But buckets exist in a specific region and you need to specify that region when you create a bucket
Refer this link to understand S3 better: https://docs.aws.amazon.com/AmazonS3/latest/dev/UsingBucket.html