In Terraform, the backend configuration, which includes details about where and how state is stored, is specified within the terraform block of your configuration. This block is the correct place to define the backend type and its configuration parameters, such as the location of the state file for a local backend or the bucket details for a remote backend like S3.
Reference = This practice is outlined in Terraform's core documentation, which provides examples and guidelines on how to configure various aspects of Terraform's behavior, including state backends .

Terraform modules are referenced by specifying a source location. This location can be a URL or a file path. However, specifying query parameters such as ?version=vl.6.0 directly within the source path is not a valid or supported method for specifying a module version in Terraform. Instead, version constraints are specified using the version argument within the module block, not as part of the source string.References = This clarification is based on Terraform's official documentation regarding module usage, which outlines the correct methods for specifying module sources and versions.

Changing the Terraform backend after performing the initial terraform apply is technically possible but strongly discouraged. This is because changing backends can lead to complexities in state management, requiring manual intervention such as state migration to ensure consistency. Terraform's documentation and best practices advise planning the backend configuration carefully before applying Terraform configurations to avoid such changes.
References = This guidance is consistent with Terraform's official documentation, which recommends careful consideration and planning of backend configurations to avoid the need for changes.

Explanation
Terraform configuration can import modules from various sources, not only from the public registry. Modules can be sourced from local file paths, Git repositories, HTTP URLs, Mercurial repositories, S3 buckets, and GCS buckets. Terraform supports a number of common conventions and syntaxes for specifying module sources, as documented in the [Module Sources] page. References = [Module Sources]

Theterraform plancommand does not update the state file. Instead, it reads the current state and the configuration files to determine what changes would be made to bring the real-world infrastructure into the desired state defined in the configuration. The plan operation is a read-only operation and does not modify the state or the infrastructure. It is theterraform applycommand that actually applies changes and updates the state file.References= Terraform's official guidelines and documentation clarify the purpose of theterraform plancommand, highlighting its role in preparing and showing an execution plan without making any changes to the actual state or infrastructure .