Is there a difference between deploying a zip file directly to AWS Lambda and using that same zip as a Lambda layer? If we are using the same content, what's the difference?

Deploying a zip file directly to Lambda means the code and its dependencies are bundled into the function’s deployment package. The entire package is uploaded, extracted, and executed in the Lambda runtime environment. A Lambda layer, on the other hand, is a separate artifact that contains libraries, custom runtimes, or other shared code. Layers are versioned and can be attached to multiple functions, allowing you to share common dependencies without duplicating them in each function’s deployment package. If you use the same content, the difference lies in packaging and reuse: a direct zip is function‑specific, while a layer is reusable across functions. Layers also reduce deployment size and can be updated independently, which can improve deployment speed and maintainability. In practice, you’d use a layer for shared libraries or binaries and a zip for the function’s unique code.