I've removed an overhead light and an access panel to the underside of the old radar mast location. As guessed at above, the flybridge structure is, from topside:
Nonskid pad/surface, or Teak, or 'gelcoat, depending upon where you look.
1/4" +/- fiberglass.
1/2" end grain balsa.
1/8" + fiberglass
1 3/4" cavity with fiberglass covered beams or hat channels glassed to the layup above.
1/8" +/- fiberglass overhead with sand-textured gelcoat, bonded to the fiberglass beams with polyester putty.
The locations of the beams are not obvious and seem to run both transverse and longitudinally (only a very small sample was seen). Where the deck passes over the salon bulkhead, the gaps between the beams are foamed full making fishing wire after the fact a tough problem. Conduits were installed at time of construction for builder-installed wiring.
Pics include a general view, a look up into the maststep access showing the conduit leading to the stern running light and the open hole where mast-mounted lights' wires had run, a look up into the space for the ceiling light - its wires pass into a conduit coming through an adjacent beam.