Terraform module need not be public and open-source. Module can be placed in -
* Local paths
* Terraform Registry
* GitHub
* Bitbucket
* Generic Git, Mercurial repositories
* HTTP URLs
* S3 buckets
* GCS buckets
https://www.terraform.io/docs/modules/sources.html

The terraform refresh command is used to reconcile the state Terraform knows about (via its state file) with the real-world infrastructure. This can be used to detect any drift from the last-known state, and to update the state file.
This does not modify infrastructure, but does modify the state file. If the state is changed, this may cause changes to occur during the next plan or apply.

https://www.terraform.io/docs/internals/debugging.html

The terraform workspace new command is used to create a new workspace.
https://www.terraform.io/docs/commands/workspace/new.html