1. The document discusses immutable infrastructure using Terraform, including defining immutable principles as not installing new software, updating servers, changing configurations, or updating code, but instead replacing infrastructure by building new images. 2. It promotes Terraform for implementing immutable infrastructure due to features like being declarative, having a state file and simple configuration language, and enabling reusable modules. 3. The document emphasizes testing infrastructure as code using tools like Terratest to test from top to bottom and prevent releasing without testing.