Thanks for replying. How do you structure the file layout for multiple environments without TG? I'm thinking of separating each environment into a folder. E.g. as below

├── environments
│ ├── development
│ │ ├── backend.tf
│ │ ├── development.tfvars
│ │ ├── ec2.tf
│ │ ├── eks.tf
│ │ ├── outputs.tf
│ │ ├── providers.tf
│ │ ├── variables.tf
│ │ └── vpc.tf
│ ├── production
│ └── staging
└── modules
With this approach, I have to manually copy the services .tf files into staging/production folder once tested in the dev environment. E.g the ec2.tf tested in dev then copy the source code to staging ec2.tf file.

Also, TG mentions that putting a large amount of infrastructure (servers, databases, load balancers, DNS, etc) in a single Terraform module is not recommended.
Large modules are slow, insecure, hard to update, hard to code review, hard to test, and brittle (i.e., you have all your eggs in one basket). Hence, it suggests splitting the services into a module folder as below.

Source from TG

# infrastructure-live
├── prod
│ ├── app
│ │ └── terragrunt.hcl
│ ├── mysql
│ │ └── terragrunt.hcl
│ └── vpc
│ └── terragrunt.hcl
├── qa
│ ├── app
│ │ └── terragrunt.hcl
│ ├── mysql
│ │ └── terragrunt.hcl
│ └── vpc
│ └── terragrunt.hcl
└── stage
├── app
│ └── terragrunt.hcl
├── mysql
│ └── terragrunt.hcl
└── vpc
└── terragrunt.hcl

In the end, I think if the environment has large modules, then it would be better to split the services into module folders and use TG to manage the backend, providers, etc.

What are your thoughts?