I agree.....an 18 model would be still under warranty so I would put it on them.
I'd bet near a 100% chance that there has been leaking and all sorts of possible problems from that.
Not to worry you, but to drive home the importance....I found a leak, not in that same spot but close, after my warranty ran out. I'm fairly sure it has been leaking since it left the factory. Rot, big repair.
I would clean it and put some self leveling caulk over that hole just to seal it till you can get it into the dealer.
but to answer the question, the roof if it's like mine, is a sandwich. Thin plywood deck under the TPO roof, sitting on top of a tubular metal frame with styrofoam insulation, and the inner ceiling skin below it all. It tapers with the rounded roof, several inches thick at the centerline, getting thinner towards the edges.
You can get a really good look at the roof structure by pulling the inner trim ring flanges off the roof vents, skylights, and/or the AC. All very easy with just a few cosmetic screws attaching the trim.