Construction costs are currently skyrocketing

Drying times should definitely be observed. Messing around for a few days now can backfire in the end. If it really can't be helped with the cable, why don't you get some mineral wool or styrofoam and stuff it into the open window? That costs a few euros and half an hour of work.

The problem isn't even that you're heating out the window. Your basement wall or the entire room cools down, the moisture present in the house condenses, and it molds.

That's why you shouldn't overdo it when ventilating in winter. Better 5 times a day for 10 minutes than once for an hour. The room must not cool down too much while airing, otherwise you (in places, preferably on the outer wall) fall below the dew point...

Which film can I use to tape off the window that keeps the heat inside? It would actually be interesting as a workaround until they are finished

Heat transfer is not the problem with regard to the small area. Air exchange should be prevented. A piece of roofing underlay / vapor barrier with fabric adhesive tape should work well. Wrap it accordingly at the penetration. Be prepared that after removal the adhesive residues will need to be cleaned, so maybe don’t use the good adhesive tape that is normally used to secure the vapor barrier – that sticks too well.

I would stuff a foam mat in there.

We have no influence on how long they wait or where they get their high voltage power from, that’s all managed by the contractor.

Excuse me, but who is the builder? That’s you, isn’t it? And I would definitely inform the contractor, who is working on your behalf and whom you pay for this, about how and what he has to do. Friendly but firmly. And if he wants to apply the finishing plaster after just one week on top of the base coat, then in my opinion that’s too soon. He can wait a few more days.

That’s exactly why there is often trouble because they work according to the contractor’s schedule instead of following the building physics regulations.

I have already seen houses where the entire plaster cracked because the finishing plaster was applied after only a few days. Where else should the moisture go? Just think about it.

For the window and those few days, you can use a simple dimpled foil and some tape. But not duct tape or packaging tape. Otherwise, you’ll get glue residue on the window.

Even easier. Just clamp a complete EPS board precisely into the window recess. For the cable, simply melt a channel along the edge with a heated screwdriver (if you don’t have a soldering iron).

You have an electric car and certainly a corresponding charging station in the garage. That should also be connected in such a way that one can draw three-phase power there, right?