how to put the same key or tag on multiple objects in an s3 bucket

Key and Tag are different things.

If you want to add tags to every object in S3, you can do that. This document shows you how to - https://docs.aws.amazon.com/AmazonS3/latest/userguide/tagging-managing.html

As far as key is concerned, S3 is an object store and every object in S3 is uniquely identified by its key, which comprises the bucket name, the prefix path and the object name. The key has to be unique for every object in S3.