Although main.tf is the standard name, it's not necessarily required. Terraform will look for any file with a .tf or .tf.json extension when running terraform commands.

https://www.terraform.io/docs/configuration/index.html#code-organization

During a terraform apply, any resources that are successfully provisioned are maintained as deployed.

On the other hand, resources that failed during the provisioning process, such as a provisioned, will be tainted to be recreated during the next run. https://www.terraform.io/docs/provisioners/index.html#creation-time-provisioners

The terraform apply command is used to apply the changes required to reach the desired state of the configuration, or the pre-determined set of actions generated by a terraform plan execution plan.

https://www.terraform.io/docs/commands/apply.html

If there is no state file associated with a Terraform configuration file, a terraform apply will create the resources defined in the configuration file. This is a normal workflow during the first terraform apply that is executed against a configuration file. This, of course, assumes that the directory has been initialized using a terraform init

https://www.terraform.io/docs/state/purpose.html

When calling a child module, values can be passed to the module to be used within the module itself.

https://www.terraform.io/docs/modules/composition.html

A local value assigns a name to an expression, so you can use it multiple times within a module without repeating it.

https://www.terraform.io/docs/configuration/locals.html

The terraform fmt command is used to rewrite Terraform configuration files to a canonical format and style. This command applies a subset of the Terraform language style conventions, along with other minor adjustments for readability.

https://www.terraform.io/docs/commands/fmt.html

The -upgrade will upgrade all previously-selected plugins to the newest version that complies with the configuration's version constraints. This will cause Terraform to ignore any selections recorded in the dependency lock file, and to take the newest available version matching the configured version constraints.

https://www.terraform.io/docs/commands/init.html#upgrade-1

All of these are examples of the benefits of using Terraform modules except where they can be stored. Modules can only be supported in certain sources found at the following link:

https://www.terraform.io/docs/modules/sources.html

In this example, the only resource that does not have an implicit or an explicit dependency is the aws_s3_bucket.customer_data. Every other resource defined in this configuration has a dependency on another resource.

https://learn.hashicorp.com/tutorials/terraform/dependencies