Module version is optional to reference a module on the Terraform Module Registry. If you omit the version constraint, Terraform will automatically use the latest available version of the module

Outside of the required_providers block, Terraform configurations can refer to providers by either their local names or their source addresses. The local name is a short name that can be used throughout the configuration, while the source address is a global identifier for the provider in the format registry.terraform.io/namespace/type.
For example, you can use either aws or registry.terraform.io/hashicorp/aws to refer to the AWS provider.

This is how Terraform manages most dependencies between resources, by using the references between them in the configuration files. For example, if resource A depends on resource B, Terraform will create resource B first and then pass its attributes to resource A.

The two steps that are required to provision new infrastructure in the Terraform workflow are init and apply. The terraform init command initializes a working directory containing Terraform configuration files.
It downloads and installs the provider plugins that are needed for the configuration, and prepares the backend for storing the state. The terraform apply command applies the changes required to reach the desired state of the configuration, as described by the resource definitions in the configuration files. It shows a plan of the proposed changes and asks for confirmation before making any changes to the infrastructure. Reference = [The Core Terraform Workflow], [Initialize a Terraform working directory with init], [Apply Terraform Configuration with apply]

These are two ways to produce a list of the IDs from a list of objects that have an attribute id, using either a for expression or a splat expression syntax.