XNAT 1.7 is very much a codebase in transition, as we are actively stepping toward an "XNAT 2.0" destination on the development roadmap. As such, there is a combination of old and new under the hood. For example, there are still Velocity templates for common report pages, but the new Admin UI is built in JSP using a YAML spec and a new-fangled widget spawner.
As such, expect to find a lot of new content and controls in this section.
XNAT build instructions for developers is much the same as the Vagrant Quick-install instructions for users, but with more customization options available.
External developers can contribute to XNAT in a number of ways: adding functions, services and data types via plugins; adding processing capabilities via pipelines; or adding connectors between their own applications and XNAT data.
With the advent of a mature XNAT API, it is easier for external developers to create a free-standing UI using whatever technology they choose, and connect all functions of that front end to XNAT services using the API. It is even possible, using CORS, to connect that front end to a remote XNAT.