The most up-to-date documentation for the Plasma Mobile stack will be found on the contributors wiki.
KDE uses the invent.kde.org GitLab instance to host source code and development work.
Plasma Mobile Shell
The Plasma Mobile shell is built on top of the Plasma stack.
The source code can be viewed at invent.kde.org/plasma/plasma-mobile.
The source code for Plasma Mobile applications by KDE can be viewed at invent.kde.org/plasma-mobile.