Explanation
https://www.terraform.io/docs/import/usage.html

Explanation
Implicit and Explicit Dependencies
By studying the resource attributes used in interpolation expressions, Terraform can automatically infer when one resource depends on another. In the example above, the reference to aws_instance.web_server.id creates an implicit dependency on the aws_instance named web_server.
Terraform uses this dependency information to determine the correct order in which to create the different resources.
# Example of Implicit Dependency
resource "aws_instance" "web_server" {
ami = "ami-b374d5a5"
instance_type = "t2.micro"
}
resource "aws_eip" "web_server_ip" {
vpc = true
instance = aws_instance.web_server.id
}
In the example above, Terraform knows that the aws_instance must be created before the aws_eip.
Implicit dependencies via interpolation expressions are the primary way to inform Terraform about these relationships, and should be used whenever possible.
Sometimes there are dependencies between resources that are not visible to Terraform. The depends_on argument is accepted by any resource and accepts a list of resources to create explicit dependencies for.
For example, perhaps an application we will run on our EC2 instance expects to use a specific Amazon S3 bucket, but that dependency is configured inside the application code and thus not visible to Terraform. In that case, we can use depends_on to explicitly declare the dependency:
# Example of Explicit Dependency
# New resource for the S3 bucket our application will use.
resource "aws_s3_bucket" "example" {
bucket = "terraform-getting-started-guide"
acl = "private"
}
# Change the aws_instance we declared earlier to now include "depends_on" resource "aws_instance" "example" { ami = "ami-2757f631" instance_type = "t2.micro"
# Tells Terraform that this EC2 instance must be created only after the
# S3 bucket has been created.
depends_on = [aws_s3_bucket.example]
}
https://learn.hashicorp.com/terraform/getting-started/dependencies.html

Explanation
For production use, you should constrain the acceptable provider versions via configuration file to ensure that new versions with breaking changes will not be automatically installed by terraform init in the future.
Example
terraform {
required_providers {
aws = ">= 2.7.0"
}
}

Explanation
While there are a ton of features that are available to open source users, many features that are part of the Enterprise offering are geared towards larger teams and enterprise functionality. To see what specific features are part of Terraform Cloud and Terraform Enterprise, check out this link. https://www.hashicorp.com/products/terraform/pricing/

Terraform state captures all dependency information, both implicit and explicit. One purpose for state is to determine the proper order to destroy resources. When resources are created all of their dependency information is stored in the state. If you destroy a resource with dependencies, Terraform can still determine the correct destroy order for all other resources because the dependencies are stored in the state.
https://www.terraform.io/docs/state/purpose.html#metadata