I was expecting some kind of answer telling me to RTFM (OK in more polite words).
But some things are related to this distro/recompilation which are not self explanatory (at least not for newbies).
Most of the things I figured out myself. But for example that you have to DISABLE Fullscreen fpr the PPSSPP Emulator to get Fullscreen was just luck to find out.
However I'm not asking for additional Emus I just wanted to know how they have to be installed (mainly the cores for Retroarch) as it seems that you can't download them in the app itself. Or that you can't start some of the Emulators with a gui (like Dolphin) to do the configuration. You first have to "run" a game and then to pause/exit them. I don't think thats a obvious procedure.
Anyway I'm now lucky with it.