Sealing transition between ground slab and wall to the soil

The site manager now thinks that a drainage would be rather disadvantageous, as it would create a water gradient. And the WU concrete in the wet clay soil does not pose a problem.

Now I would like to get at least a second opinion, because of course I want to have a foundation that is as stable as possible in the long term. Can a drainage also be disadvantageous in clay soil? Or am I not making a mistake with a drainage all around the floor slab?

In general, I wouldn't know in what way a drainage could negatively affect the construction. The question is rather whether a drainage is permissible, or where the water is led to and whether the effort and costs are even worthwhile in this case. You have a WU floor slab (which is actually not necessary in 95% of non-basement single-family houses) and I assume no groundwater. A drainage would therefore only drain rainwater/surface water. As long as the water does not flow onto the floor slab, I see no necessity for the drainage.

A terrain section with the house would also be advantageous to better assess the situation.

How was the building founded?

Thank you very much for your reply. The terrain profile can be easily described - no hills or slopes, everything flat. It is purely about the fact that the clay soil always contains a lot of moisture that does not seep away. There is tendentially always dampness (no waterlogging, as no groundwater is reached within 80cm) present here. And for that, I was rather thinking of ventilation through [Nobbenbahnfolie] and drainage pipe (drained).

we have similar soil and a drainage lies in the gravel bed under the BP (done by the concrete builders). Additionally, we have a ring drainage (done by ourselves). Usually, this goes into a soakaway because it is forbidden to discharge surface water from one's own property into the sewer system (here)

This moisture in the clay soil is actually quite normal. As the construction manager already said, you have a slab against constantly pressing water. If you only had a slab against ground moisture, then you would have had to keep the water away from the slab using gravel cushions or drainage. The drainage would also only drain off any rainfall faster. You cannot get rid of the ground moisture that way. There really cannot be any air circulation there. But as already stated, I do not see any disadvantages (besides the additional costs/effort).