i loaded a kde-base-workspace package. i was looking for the implementation of the solid api. but i dont get it ):
there is a /lib/solid folder and a /solid folder both containing some identical named files and are cross-including each other (i think).

the goal was it so be able to write some functionality by my own, but i dont know what to do. there weren't any comments or examples. so could somebody help? questions are for example:
the checkout wheather things like hal/networkmanager are installed or not.
abraction techniques and where to implement this
the meaning of the dir-hirarchy.

Solid is part of the KDE Libraries. The backends are stored in KDE Workspace, which is what you found. All needed information can be found at api.kde.org with regards to the API.