How do S3 work in the back end?

Amazon S3 uses a multi-layered security approach in the backend to protect your data and buckets. Here's how it works:

Bucket Security: By default, all S3 buckets and objects are private - only the root user of the AWS account that created the bucket and authorized IAM users can access them. S3 provides several mechanisms to secure buckets:

• Encryption: S3 automatically encrypts all object uploads using server-side encryption, protecting your data at rest
• Block Public Access: You can block public access to all objects at the bucket or account level to prevent accidental exposure
• Access Control: Buckets use JSON-formatted bucket policies and identity-based IAM policies to define who can access what resources and what actions they can perform

Access Management: S3 manages access through several tools:

• Bucket policies attached directly to buckets
• IAM policies attached to users, groups, or roles
• S3 Access Grants for managing permissions at scale, including for corporate directory users
• Access Points with customized permissions and network controls

Authentication Flow: When users or applications request access to S3 data, S3 validates the request against existing policies and grants. For features like S3 Access Grants, the service vends temporary credentials to authorized requesters, who then use those credentials to access the data.

Source Code: AWS does not provide the source code for S3's backend infrastructure. S3 is a proprietary managed service, and its internal implementation details and code are not publicly available. However, AWS does provide SDKs and APIs that you can use to interact with S3 programmatically.
Sources
Amazon S3 Security Features - Amazon Web Services
Access control in Amazon S3 - Amazon Simple Storage Service
Getting started with S3 Access Grants - Amazon Simple Storage Service