How did you teach yourself lambda functions and documentsdb

Take a look at this: Creating a REST API for Amazon DocumentDB (with MongoDB compatibility) with Amazon API Gateway and AWS Lambda.

I would recommend taking the free AWS provided lambda course it will walk you through many of the basics. There also many How To Guides Although not identical there is Example Project of using Lambda to connect DynamoDB

Although covered in training I like to make sure I have a machine configured correctly for development before I start.
There are MANY tools for .NET Development with AWS Note some of this is covered in the above training.

I install the following:

• Install Visual Studio Tool Kit in Visual Studio
• Install Visual Studio Tool Kit in Visual Studio Code using Extensions Panel
• Install AWS Command Line Interface (AWS CLI) and make sure you can authenticate
• DotNet Gobal Tool for AWS Lambda dotnet tool install -g Amazon.Lambda.Tools

Although .NET 7 is the latest I find it easier to use .NET 6 initially, as it is a built in already supported runtime. Once you have execution with .NET 6 you can look at a custom runtime image to do .NET 7.

I typically test my lambdas locally before moving to AWS. To do that I use the Mock Lambda Test Tool found in the tool bar of Visual Studio. It will launch a browser window where you can send whatever you want to your lambda function and be able to step through it.

There is much more, I would encourage you to spend some time exploring the .NET on AWS Site to become familiar with the resources available.