AWS's Unity SDK with modified example files.
Unity 2018.3.14f1 (or any version of Unity 2018 should work). A private S3 bucket. Go to this link for a tutorial on how to make one: https://uberact.com/blog/2019/5/3/tutorial-setting-up-aws-s3-and-installing-the-aws-mobile-sdk-for-unity-using-the-s3-example A demo AssetBundle can be found in Assets/AssetBundles.
Open the project in Unity. Select the 2D view. Find the example scene and open it.
Select the S3 GameObject and enter the information on S3Example.cs. Go to the link above if you need help finding the information.