OK I'll bite: What does starship add here? It's a very large re-useable faring that carries a stage + payload into orbit? Why not attach to the top of FH?
Starship doesn’t add anything. Starship removes complexity by sheer brute force, cheaply due to reusabilty. You can architecture a mission with ridiculous (before starship, that is) mass and volume per dollar budgets.
Falcon Heavy can lift a lot of mass but unless you’re launching tungsten into orbit you can’t fit interesting things into the fairing.
FH is already very thin and tall, probably not practical to stick another stage on top even if it could work payload wise. Besides that, SLS and New Glenn are the only ones with comparable lift capacities, SLS is way too expensive for the capability and the massive SRBs make the ride pretty rough, New Glenn is probably a feasible alternative, although probably more expensive than Starship.
Why: FH + Starship w/( rocket + payload)
And not: FH + (rocket + payload)