KDE software is hosted on infrastructure managed by the KDE Sysadmins.
Every accounts in the KDE infrastructure are synchronized in a same system. It is called KDE Identity.
The KDE projects are mostly using Git. The repositories can be browsed on our QuickGit and managed on KDE Projects.
Find guidelines here.
Some projects such as i18n (to localize the programs) or some websites still use SVN. The repository can be browsed on https://websvn.kde.org/. Some guidelines could be found here.
Find out how to get commit access.
The code is reviewed by the community before being pushed on a repository.
The current used system is Review Board. However projects are heavily encouraged to move to the new system: Phabricator.