natural weed killer before topsoil is spread

That is all unnatural on a surface level and damages the soil.

I had written that it is not good and perhaps even prohibited, but apparently not clearly enough. Some even say that glyphosate-containing products are sometimes better for the soil than home remedies like vinegar, salt, etc. Since you only have to apply it once and others need to be repeated, and you acidify and oversalt the soil too much. But whether you want glyphosate in your garden is up to each person to decide.

The "Boden" is a layer of soil that is not even 5 cm thick. I want to clear it of weeds so that they cannot penetrate through the topsoil that will be piled on it. I will get the topsoil only at the beginning of August.

I think that the Earth will not be your problem. The 'new' Mother Earth will contain enough seeds.

But exactly this soil that has now been removed is the problem!

- Do nothing
- black foil
- sift out roots and then cover

I wouldn't do anything at all. Much does not penetrate the topsoil, and it also brings quite a bit with it.

Except for the part where the lawn didn’t grow, weeds are not a problem for us. We haven't done anything at all.
The topsoil brings along the plants you plant, many things come flying in. I have a hazelnut invasion. Many thanks to the squirrels and the jay.

Yes, exactly this soil that has now been removed is the problem!

No, that was not what meant. The topsoil to be delivered still contains countless new seeds. Where you lay and maintain lawn, weeds will disappear very quickly anyway. Now and then you have to dig something out, and in beds you can’t avoid hoeing weeds. Otherwise, everything has been said here: screening and black foil (but that also kills beneficials like earthworms) or sowing dense green manure (by the way, I use mustard seed, which is fast, inexpensive, and even decorative). The seeds contained deep in the soil die off on their own.