Insulation of the attic of an old country house

Hello dear house builders,

Recently, my girlfriend and I became proud owners of an old country house with quite a bit of land. The ground floor is finished and was mostly already renovated when we bought the property.
Now we are venturing into the expansion of the attic. However, I have some questions about this where I would like to rely on your expert opinion.

The attic is currently not insulated at all. Implementing this is at the top of the priority list.

The wall insulation is to be done as interior insulation. Exterior wall - insulation - interior cladding (drywall sheets).
The structural engineer informed us that he envisions at least 14cm of insulation with WLB035, with interior insulation including at least 4cm of ventilation space. Supply air from below and exhaust air in the cavities. So far, I have understood the principle behind this. I'm just not really clear on the exact construction.
How does the supply air get in from below? Will I then always have a visible gap between the floor covering and the drywall?
And can I do without a vapor barrier when using ventilation? Or does the setup exterior wall - ventilation 4cm (battens) - mineral wool - vapor barrier - drywall make the most sense?

Unfortunately, the roof only has an underlay foil in the extension. For a large part of the roof, you can look directly from below onto the concrete roof tiles.
Covering and installing an underlay foil is hardly an option due to the effort and cost. I have read several times that an alternative looks as follows:
Install insulation with at least 4-5cm of air space to the roof tiles, then place the vapor barrier on the inside and in front of that the interior cladding (here we also want to use drywall sheets). Basically, this is nothing else than ventilation on the roof. Does anyone have experience with this? The roof pitch is 42°.



Thank you in advance to all helpers.

Regarding interior insulation, I recommend a real expert. Interior insulation can become very, very critical with mold, etc.

Roof: you fill the space between the beams with mineral wool, but leave a 2cm gap to the roof tiles. Then you nail beams/laths crosswise over it again and fill up there as well. Then comes a climate membrane and plasterboard. Done. With insulation, the basic rule is: more is always better.
Regarding heat protection: mass is power.

Put differently: The supply air naturally does not come from inside but from outside.

Thank you very much for the answers. I still do not quite understand the ventilation. How does the supply air from outside get between the insulation and the wall? The masonry is closed.

It's about the roof, right? The roof covering is not airtight.

The interior insulation of the wall must definitely be planned and explained to you by a professional. I am self-taught and do a lot myself, but even I am extremely cautious with interior insulation, as it can very quickly lead to massive structural damage and mold.