Virtually all fiberglass boats built today have coring; either balsa, plywood or foam; above the water line. Balsa is the most prevalent core.
Balsa and plywood core can get water inside and rot if the fiberglass layer above it is breached. This is often due to stressed stanchions that break the caulk seal or owner or even boatyard installed deck fittings done without removing and filling the underlying core.
From my limited experience, other than core material, there isn't anything different about how one builder builds them vs another.