None of the above methods prevent credentials from being stored in the state file. Terraform stores the provider configuration in the state file, which may include sensitive information such as credentials. This is a potential security risk and should be avoided if possible. To prevent credentials from being stored in the state file, you can use one of the following methods:
Use environment variables to pass credentials to the provider. This way, the credentials are not part of the provider configuration and are not stored in the state file. However, this method may not work for some providers that require credentials to be set in the provider block.
Use dynamic credentials to authenticate with your cloud provider. This way, Terraform Cloud or Enterprise will request temporary credentials from your cloud provider for each run and use them to provision your resources. The credentials are not stored in the state file and are revoked after the run is completed. This method is supported for AWS, Google Cloud Platform, Azure, and Vault. References = : [Sensitive Values in State] : Authenticate providers with dynamic credentials

To import existing resources into Terraform, you need to do two things1:
Write a resource configuration block for each resource, matching the type and name used in your state file.
Run terraform import for each resource, specifying its address and ID. There is no such command as terraform Import-gcp, and provisioning new VMs with the same names will not import them into Terraform.

Not all standard backend types support state locking and remote operations like plan, apply, and destroy. For example, the local backend does not support remote operations and state locking. State locking is a feature that ensures that no two users can make changes to the state file at the same time, which is crucial for preventing race conditions. Remote operations allow running Terraform commands on a remote server, which is supported by some backends like remote or consul, but not all.
References:
Terraform documentation on backends: Terraform Backends
Detailed backend support: Terraform Backend Types

These are examples of infrastructure as code (IaC), which is a practice of managing and provisioning infrastructure through machine-readable definition files, rather than physical hardware configuration or interactive configuration tools.

Sensitive values such as API tokens should be stored in a secure way, either in Terraform Cloud variables marked as sensitive or in HashiCorp Vault. Storing secrets in version control systems or plaintext files is not recommended.
References:
Terraform Cloud Environment Variables