Hi Nitin,
Yes OutSystems PaaS is hosted on AWS. But that does not mean that all AWS services are available for you in the OutSystems AWS VPC.
If you want to use AWS S3 Buckets, you will need to have your own AWS account, you cannot request this on your OutSystems AWS account.
If you create your own AWS account and use services from AWS they can be accessible by non-OutSystems applications if you setup proper permissions and access control. Furthermore your other applications need to be able to access the internet.
Regards,
Daniel