DynamoDB is a low latency, serverless-compatible database. It can handle scale you'll probably never see and comes with encryption-at-rest, backup-and-restore, and a transparent caching layer to eek out faster-than-millisecond performance. It's fully managed, which means you're not worrying about servers, containers, or scaling and comes in a variety of pricing models to fit your use case, such as on-demand, provisioned and reserved. If you know your access patterns and can fit them into DynamoDB, you'll be able to scale easily.
When AWS moved from Oracle to their own databases, the low-latency services were migrated to DynamoDB (https://aws.amazon.com/blogs/aws/migration-complete-amazons-consumer-business-just-turned-off-its-final-oracle-database/)
DocumentClient is a high-level client. There are low-level clients that we won't be diving in to. There are even *higher* level clients that we also won't be diving in to. Three types of clients are: Low-Level Interfaces, Document Interfaces, and Object Persistence interface (Java+C# only)
Approaches to building different kinds of systems. Depends how long they are, might be courses in and of themselves.