welcome 🙂
We really need more information. A list of what you have tried and would like to try would be helpful as well. Additionally, it would be good to know whether you are wanting to use the AWS S3 Bucket to store only media or more.
From a developer standpoint:
Multi-site implementations give each site an ID. You can get the site’s ID with the function get_current_blog_id() and then use that to tell your plugin which hard-coded AWS S3 Bucket to use.
Alternatively, you can make the plugin have an options page where you can add the Bucket credentials and let it handle authentication from there.
Since, normally, plugin settings and media libraries are not shared between network sites you would likely be able to find a plugin that will suit your needs. This one seems popular: https://wordpress.org/plugins/amazon-s3-and-cloudfront/