Using Git for FreeBSD Development Today
The latest document about using git for tracking or developing FreeBSD is at https://github.com/bsdimp/freebsd-git-docs It will be sorted out as a chapter in handbook and committer's guide.
The way of using git to hack on FreeBSD in svn is documented here: GitWorkflow. (Mostly obsoleted by migrating to git.)
https://git.FreeBSD.org is the official mirror for the public, currently has doc and src trees.
The FreeBSD project page on Github has the official mirrors for src, doc, and ports repositories.
Git for FreeBSD Development in the Future
In 2019, Core chartered a Git Working Group (May 2019 DevSummit announcement (Youtube); Slides from same, p. 49+) to work out the implementation details of using Git as the primary VCS for FreeBSD development.