Since the entire Nuxeo code base is open source, users and developers alike are welcome to download that code base, examine it and, if they wish, use that source to build the equivalent of the downloadable and installable Nuxeo packages for themselves (provided they've also installed the required development tools).
This set of pages explains how to get started checking out the code base and, eventually, how to build an installable Nuxeo DM package for yourself. Getting into actually modifying the code base and submitting your changes back to the Nuxeo development team is a more complicated subject and is left for a later section on how to become what is called a Nuxeo "core developer." For the time being, this section describes how to treat the source code in a "read-only" fashion.
The majority of the child pages here explain how to check out and build Nuxeo's flagship DM (Document Management) package. The final section covers a number of other Nuxeo packages that can be built from the source in a similar fashion.