The installer package houses all of the types, classes, and utilities for building a Blitz Recipe. A Blitz recipe is effectively just a list of steps, represented as an array of objects that conform to one of the types in the Executor union type (NewFileExecutor | AddDependencyExecutor | FileTransformExecutor). These executors are processed by the framework, executed interactively by the user, and ultimately run to install new packages to an existing Blitz app.
You can find the implementation of all Executors in the executors/ directory, stock transforms that we'll be supplying to authors in transforms/, and various utilities in utils/, including a paths utility that the user can access for common paths to modify such as _document.tsx.