Neither am I sure to understand the question.
I've found no definitive information source. I suppose that the copper foils are laminated on the partly cured glass+epoxy composite ("prepreg"), and final curing of the epoxy glues the copper. Though, it seems that much work has been done to improve the adherence of the copper clad, so I can't exclude that an additional glue layer is used.
A manufacturer's website suggests that the copper film is initially thicker, and gets thinned (to 35µm or 70µm for instance) after it holds on the glass+expoxy.