I just read in the Adobe manual that trapping presets in InDesign take effect when the document is printed, not before. But it is not clear to me. For example: I create a document and want to prepare it for the printhouse, and i need to export it as composite PDF, so the printhouse can do its job. Are the trapping presets applied to this exported composite PDF? Or does it only take effect when I actually use the Print command in InDesign?
That is correct. For trapping to be applied you need to File > Print to have it applied.
However, InDesign’s Attributes palette lets you control overprinting. Or you can even use InDesign’s built-in trapping feature by printing separations to disk.