Each folder has a README.md with the instructions about what you need to do.
the first code you need to execute is located in s3 folder, as it is required to have the bucket created in order to save the remote terraform config.
Then you have to execute the code that it's located in aws folder. There you can find instructions to build the infrastructure where the cluster will run.
Then you should create the mongodb images in mongodb/packer folder and start the instances in mongodb/terraform folder.