The solution is a POC for following
Let's consider - there are 3 AWS account.
Account A is the owner of a S3 bucket called demo-bucket
Account B has permission to write data into (say, 1 million objects)demo-bucket of Account A
Now Account C is intended to read the data from demo-bucket.
Since, Account B writes the data into demo-bucket, so they become the owner of the data. Where as Account A is the owner of the bucket but it can not provide access to objects of the bucket to Account C, since it's not the owner of objects.
The problem can be solved by several ways, here is one basic solution how it can be achievable using AWS lambda function, using serverless
In Account A we can run lambda function which will copy the object from one directory to other directory inside the same bucket or we can copy it completely different bucket. In this solution - I have used different bucket.
So, when the objects will land into demo-bucket, the objects will be copied into another bucket demo-bucket-1. Now Account A is the owner of bucket and each object inside bucket. So you can control the access through bucket policy. The access can be granted to N no. of accounts through bucket policy.