natural weed killer before topsoil is spread

Hello
Today, the excavator removed the top layer of soil including weeds and roots and stored it on the side.
Later on, topsoil is to be piled onto this layer.
To prevent new weeds from getting a chance, I would like to use natural weed killer now so that it has no chance to penetrate through the topsoil later.
What do you recommend? Rock salt?

If you google it, vinegar, salt, and baking soda come up. But these agents are not good either, if not even prohibited. Whereas some ready-made glyphosate-free herbicides even contain acetic acid as an active ingredient.

Possibly cover with a tarpaulin?

Black tarpaulin is a good idea because:

thermal weed control is based on a biochemical principle: most proteins denature at a temperature of about 42 °C.

If a "core temperature" of 42° can be reached in the pile, everything should be destroyed. Compost heaps often become significantly hotter (70 - 80°).

please no salt - [Folie] is probably the gentlest solution. What area is it and for what period? If there is such a huge [Folie], then no rain gets into the ground at that spot and during heavy rain it floods.... the neighbors...?

When you google it, you get vinegar, salt, and baking soda

That is all unnatural on the surface and damages the soil. You plant cover crops like the farmer does, such as rapeseed, lupine, and similar. That protects the soil and new nutrients are formed. Taproots loosen the soil.