The film's novelisation indicates that the only people on the roof of the Steadfast were Finn and Jannah. The feature you're looking at is Jannah's distinctive curved bow.
Finn almost lost his grip when he saw the Falcon’s hull rise below him.
“Jannah!” he yelled, surging with hope.
They would have to time this just right. They’d have a few steps of running room along the side of the comm tower, and then nothing but air.
They gripped hands. Not just yet…Now! They took off running as Lando guided the hull closer. With all their might, they leapt…just as the Steadfast finally succumbed to gravity and dropped toward oblivion.
They landed hard on the Falcon’s hull; Finn’s ankle twisted. A hatch opened revealing Chewbacca, who waved at them to hurry. They sprinted across the top of the Falcon, Finn’s ankle screaming with each step, and finally dropped into the hold.
“Chewie, you got the kids?” Lando hollered from the cockpit.
Chewie roared confirmation, and the Falcon sped away.
Finn collapsed against the wall. He couldn’t believe they’d survived.
Star Wars: The Rise of Skywalker: Expanded Edition