Source Code
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.
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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.
Applications
The source code for Plasma Mobile applications by KDE can be viewed at invent.kde.org/plasma-mobile.