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

Explanation
By default, Terraform will clone and use the default branch (referenced by HEAD) in the selected repository.
You can override this using the ref argument:
module "vpc" {
source = "git::https://example.com/vpc.git?ref=v1.2.0"
}
The value of the ref argument can be any reference that would be accepted by the git checkout command, including branch and tag names.
https://www.terraform.io/docs/modules/sources.html

If supported by your backend, Terraform will lock your state for all operations that could write state. This prevents others from acquiring the lock and potentially corrupting your state.
State locking happens automatically on all operations that could write state. You won't see any message that it is happening. If state locking fails, Terraform will not continue. You can disable state locking for most commands with the -lock flag but it is not recommended.
If acquiring the lock is taking longer than expected, Terraform will output a status message. If Terraform doesn't output a message, state locking is still occurring if your backend supports it.
Not all backends support locking. Please view the list of backend types for details on whether a backend supports locking or not.
https://www.terraform.io/docs/state/locking.html

Explanation
For local state, Terraform stores the workspace states in a directory called terraform.tfstate.d.
https://www.terraform.io/docs/state/workspaces.html#workspace-internals

Providers usually require some configuration of their own to specify endpoint URLs, regions, authentication settings.
Providers Initialization can be done by either explicitly via a provider block or by adding a resource from that provide
https://www.terraform.io/docs/configuration/providers.html