So, in The Last Jedi we see
a Rebel bomber destroy a First Order dreadnought by mass bombing a circle on the top of the ship
So what was that under it, that could do that? It's not near the engines. Was that
the reactor for the big guns?
The bomber was targeting the Fulminatrix's reactor:
BATTLE OF D'QAR
Poe Dameron leads the attack to disable the First Order dreadnought, an unmistakable fleet killer bristling with huge cannons. He flies ahead to singlehandedly take out the many anti-starfighter defenses on the dreadnought's hull, while squadrons of X-wings and A-wings escort bomber craft into their drop zones. The ponderous bombers are vulnerable to enemy fighters and many are destroyed by TIE craft. Poe's decision to continue the attack results in the loss of all but one bomber, which delivers its entire payload onto the dreadnought's reactor. The Fulminatrix erupts in flames and the Resistance fleet jumps to lightspeed.
The Rise of Skywalker: The Visual Dictionary, page 22: Resistance Retreat; emphasis added
Battles that Changed the Galaxy elaborates on its effects on the dreadnought, though doesn't mention the reactor:
Dreadnought Carnage Only a single bomber dropped its payload, but that was enough to tear the First Order dreadnought apart. As explosions stripped through the Fulminatrix's hull, more than 200,000 of its crew were killed, including Captain Canady, who accepted his fate and did not try to escape.
Battles that Changed the Galaxy, page 221