Variables declared within a module are only accessible within that module, unless they are explicitly exposed as output values1.

A module cannot always refer to all variables declared in its parent module, as it needs to explicitly declare input variables and assign values to them from the parent module's arguments. A module cannot access the parent module's variables directly, unless they are passed as input arguments.

You do not need to use different Terraform commands depending on the cloud provider you use. Terraform commands are consistent across different providers, as they operate on the Terraform configuration files and state files, not on the provider APIs directly.

Before using a remote backend in Terraform, it is mandatory to run terraform init. This command initializes a Terraform working directory, which includes configuring the backend. If a remote backend is specified, terraform init will set up the working directory to use it, including copying any existing state to the remote backend if necessary.
Reference = This principle is a fundamental part of working with Terraform and its backends, as outlined in general Terraform documentation and best practices. The specific HashiCorp Terraform Associate (003) study materials in the provided files did not include direct references to this information.

You can configure a backend in your Terraform code before initializing it. Initializing a backend will store the state file remotely and enable features like locking and workspaces. Reference = [Terraform Backends]