The primary goal of Aura is to provide high-quality, well-tested, standards-compliant, decoupled libraries that can be used in any codebase. This means you can use as much or as little of the project as you like.
The Aura project centers around a collection of independent packages. Each package is self-contained and has only the things it needs for its core purpose. None of the packages depends on any of the other; you can download and use each of the packages on its own.
The Aura System provides a full-stack Aura framework built around Aura library packages. If you are looking for standalone versions of a package look into the package documentation.