Mailing the story:
How AWS Lambda Serverless Works (with a whole bunch of Amazon Linux servers...)
At the top of the logical view of a worker host is customer code, which is what Lambda runs for its customers. Mesrobian commented that Lambda supports a number of languages and different runtimes including node, Python, Java, C# and more. Underneath the runtime is a sandbox that hosts the runtime and underneath that is a guest operating system that is running Amazon Linux.