1. I know it says surface "body" however it's really not
surface body have no defined thickness so don't exist but they show a visual representation
To cut whatever/wherever your trying to cut on that bonnet, you need to add a defined thickness to the surface "body" to make it and actual "body"
you basically do nothing to surface bodies until they actually "exist"
Wow. Where did you learn this rubbish. May I suggest you not comment on things that you don't know.
Surface bodies are true Fusion objects and are very useful as construction geometry faces as and when required (See below). Only true thing you said was that they have no thickness.
@sarik0031 Extrude the grill curves to intersect the front edge of the bonnet. Use the extrusion as your cutter (green), with the Surface > Modify > Trim Tool, and the offcut (reddish) is disposed of in the same operation.

Before OK.
After OK, with the cutter hidden Front view.
Might help....