Sealing on Styrodur insulation?

How would it be done correctly then? Formwork, pouring, sealing the concrete and then gluing the insulation to the floor slab? What about the insulation under the floor slab? There is no way to seal it there, right?

Correct. The floor slab is also not sealed underneath (except with the [black tank]). The sealing of the floor slab takes place on the floor slab and must be brought up to the [horizontal barrier] beneath the lowest stone. The external sealing must also be brought up to this [horizontal barrier]. This creates a closed system.

What is the damage pattern? So the thing with "building on the insulation" is - if not taken into account - clear even to me. Moisture thus penetrates behind the insulation and possibly soaks into the base slab. Doesn't the horizontal barrier under the first row of stones also come into effect here?

It's not so much about the damage pattern here, but rather about what is permissible and what is not permissible.

Furthermore, you should ensure that the new sealing is carried out according to regulations up to 30 cm above the planned terrain.

@Ulrich: you should explain the load transfer to me again. It is common today that insulation is installed at the transitions from the base slab to the wall, and Styrodur is also used on the floor slabs to prevent thermal bridges. Usually, the last brick on which the filigree ceiling ultimately rests, the ceiling, and the first brick of the new row are made weaker to avoid thermal bridges, and in front of it a 6-10 cm thick Styrodur board is installed. If that is actually not allowed, then you can demolish every house built today in solid construction because it is done that way everywhere.