AWS S3 bucket policy example - one bucket (public, to a cloudfront distribution) and restrict permissions on some folders by username.

policy.json

{
    "Version": "2012-10-17",
    "Id": "Policy1556318075116",
    "Statement": [
        {
            "Sid": "Stmt1556317630862",
            "Effect": "Allow",
            "Principal": "*",
            "Action": "s3:GetObject",
            "Resource": "arn:aws:s3:::cdn.example/videos/*"
        },
        {
            "Sid": "Stmt1556317630862",
            "Effect": "Allow",
            "Principal": "*",
            "Action": "s3:GetObject",
            "Resource": "arn:aws:s3:::cdn.example/images/*"
        },
        {
            "Sid": "Stmt1556317630862",
            "Effect": "Deny",
            "Principal": "*",
            "Action": "s3:*",
            "Resource": [
                "arn:aws:s3:::cdn.example/xml/*",
                "arn:aws:s3:::cdn.example/private/*"
            ],
            "Condition": {
                "ForAnyValue:StringNotEquals": {
                    "aws:username": "my-app-user"
                }
            }
        }
    ]
}