The version control system Git is used by the developers to keep track of changes to the sources of BZFlag.
The git repo is hosted by github. You can find an overview on this page.
To get a copy of the sources for BZFlag you need git. Once installed use
git clone https://github.com/BZFlag-Dev/bzflag.git
to clone the repository. This creates the directory bzflag which contains all the sources.