So in our project we are using dynamodb. We had to load billions of data and now its so expensive. Are the developers at fault? I'm not sure who decided to use dynamodb for the project.

If the client didn't make an effort to understand the cost of running the project... idk, that's mostly on them, but developers should make an effort to h the client be aware of the costs of the cloud (and the benefit of not having to maintain that infrastructure).