It turns out that the solution to my problem is quite trivial, I have to go to the :
Dashboard → Pages & Themes → Page Types → Output / Defaults and Output
select there again my page scheme. i will be taken to the block wizard or something like that. there i give a squat with a plus and select / find Composer Control in the list. I drag it to the desired location and in the properties set the block to which it should refer (content, img, etc)
I don’t understand why this is so hidden still additionally behind the website template… but anyway thank you very much for your help and sorry for the confusion 