Insulation of the top floor ceiling (cold attic)

Ok, so I should always include a slight ventilation there.

let a [Isolierer/Spezialist] take a look, remote consultation can cause you serious building damage

I unfortunately do not understand the purpose of the project. What speaks against Styrofoam and pressed chipboard panels on top for walking on? I have never heard of embedding it in concrete...

What speaks against putting concrete or screed on the Styrofoam?

Too complicated and too expensive. Simply lay a walkable composite board on the concrete floor. Pay attention to the insulation thickness and comply with the Energy Saving Ordinance. A vapor barrier/seal is not required (provided it is actually pure concrete). Otherwise, leave the attic open, do not stick or seal anything to the rafters, proper ventilation MUST be ensured. It’s done in no time and fits.

Ok thank you very much
I will talk to the [Isolierer] and [Zimmermann] here to see what makes sense. I don't want any botched work here.

the concreting came from the builder. It is supposed to better retain heat in the summer due to the mass.