This command will check if all code in a Terraform configuration that references multiple modules is properly formatted without making changes, and will return a non-zero exit code if any files need formatting. The other commands will either make changes, list the files that need formatting, or not check the modules.

The Terraform binary version and provider versions do not have to match each other in a single configuration.
Terraform allows you to specify provider version constraints in the configuration's terraform block, which can be different from the Terraform binary version1. Terraform will use the newest version of the provider that meets the configuration's version constraints2. You can also use the dependency lock file to ensure Terraform is using the correct provider version3. References =
*1: Providers - Configuration Language | Terraform | HashiCorp Developer
*2: Multiple provider versions with Terraform - Stack Overflow
*3: Lock and upgrade provider versions | Terraform - HashiCorp Developer

This is a secure way to inject sensitive variables into your Terraform run, as they will not be stored in any file or source code repository. You can also use environment variables or variable files with encryption to pass sensitive variables to Terraform.

Terraform allows usingprivate module sourceshosted in:
C (Internally hosted VCS platforms, e.g., GitLab, Bitbucket, GitHub Enterprise)# D (Private GitHub repositories)# A (Public GitHub repository)#-GitHubis supported, but apublic repo is not "private".
B (Public Terraform Registry)#-The public registryis not a private source.
Official Terraform Documentation Reference:
Terraform Module Sources

The reason why the apply fails after adding a new provider to the configuration and immediately running terraform apply in the CD using the local backend is because Terraform needs to install the necessary plugins first. Terraform providers are plugins that Terraform uses to interact with various cloud services and other APIs. Each provider has a source address that determines where to downloadit from. When Terraform encounters a new provider in the configuration, it needs to run terraform init first to install the provider plugins in a local directory. Without the plugins, Terraform cannot communicate with the provider and perform the desired actions. References = [Provider Requirements], [Provider Installation]