In the ending of Godzilla vs. Kong, Kong and Godzilla destroy Mechagodzilla and part ways, with Godzilla returning to the ocean and Kong staying in the Hollow Earth. Apex's plan to control the Titans fails, and Monarch builds an observation post in the Hollow Earth.
The film ends with a mutual acknowledgment instead of a winner, because Godzilla beats Kong in Hong Kong but both are needed to beat Mechagodzilla. In my opinion, that is the smartest choice in the script: the title fight settles nothing, and the real victory comes when the two Titans stop fighting each other. For the strength debate itself, the Godzilla vs. Kong: who wins page covers it.
The last act is built from a chain of specific beats. Godzilla leaves Kong badly hurt after the Hong Kong fight, and the humans bring Kong back with an electrical shock from Nathan Lind. Josh Valentine disrupts the machine's controls, and Godzilla powers up the glowing axe, made from a Godzilla dorsal plate, with his atomic breath. Kong then uses that axe to destroy Mechagodzilla. The axe matters because it makes the kill a shared act: Kong swings it, but Godzilla supplies the power.
Mechagodzilla runs on a link to the severed head of King Ghidorah, takes on the will of King Ghidorah and kills Walter Simmons before Kong destroys it. That ties this film back to Godzilla: King of the Monsters, the third film in both release order and chronological order. In the series, this is the point where the Apex plot answers Simmons's earlier claim that humans need a way to stop the Titans, and it answers it by showing that the machine built for that job turns on its own creator.
Compared with the rest of the series, the ending is also a reset of territory. Kong: Skull Island is first chronologically and Godzilla (2014) second, and both treat Kong's and Godzilla's worlds as separate. This film closes the same way: Godzilla goes back to the ocean, and Kong is now the ruler of the Hollow Earth, where Monarch builds its observation post. That leaves the two Titans in separate territories and sets up further Monarch work below the surface. The thread leads into Godzilla x Kong: The New Empire, the fifth film.
The production record gives context for the finish. The Numbers lists a $155 million budget, and Box Office Mojo reports $470.1 million worldwide. Film Music Reporter notes the film was originally scheduled for November 20, 2020, and that Tom Holkenborg was announced as composer in June 2020, so the score was commissioned well before the March 31, 2021 release.