How to separate some faces from a mesh ?

Hello,

I am building a car, and currently creating the openings (for headlights, exhausts, etc).
I do this by deleting faces.

But I would prefer if I could somehow “keep” the deleted faces, to use them e.g. for the headlights,
so that it matches exactly the opening. Otherwise its kind of tought to match the opening exactly.

So is there some way to “detach” faces from a mesh ?

Thanks a lot in advance,
Dimitris

Select the faces and in edit mode press P to separate to a new object
Y splits them off but does not make them a new object