In the ending of Godzilla: King of the Monsters, Godzilla destroys King Ghidorah with a powerful atomic pulse after Mothra sacrifices herself so that Godzilla can unleash a stronger blast, and Emma Russell also dies in the blast. Emma stays at the Orca to distract Ghidorah while her family escapes. The surviving Titans, Rodan among them, bow to Godzilla as ruler.
Why does the bowing matter? The bowing shows the Titans follow whoever wins the fight. The film shows this twice: they bow to Ghidorah after Godzilla appears to fall in the military weapon scene, then bow to Godzilla once he returns. The Orca complicates this, since Emma built her plan around calling the Titans rather than waiting for a winner, yet the final order is still settled by a fight. Rodan is the clearest example: Ghidorah defeats him in the middle of the film, Mothra defeats him again in Boston, and he still ends the film bowing to Godzilla.
The sacrifices also read as a relay. Dr. Ishiro Serizawa's nuclear device restores Godzilla, Mothra's energy powers the finishing blast, and Emma buys the time that lets her family leave. Dr. Vivienne Graham's death at Ghidorah's release in Antarctica opens that chain, so four Monarch and Titan characters die before the final pulse lands. In my opinion, the most telling choice is that Madison is the one who carries the Orca to Boston, which hands the family's arc from the mother who started the crisis to the daughter who helps end it.
The setting was built to carry that weight. According to the Motion Picture Association's 2019 piece on production designer Scott Chambliss, the Boston finale recreated Fenway Park on a back lot baseball field, with broadcast booth and corridor sets built on soundstages in both intact and destroyed versions. That explains why the park can be shown whole when Ghidorah follows Madison there and wrecked by the time the fight ends. The same source describes separate Boston and San Francisco ruin sets on the back lot, so the aftermath was physical space, not only digital effects.
The last scenes point forward. Titans spread across the planet, and the film hints at a possible second Mothra egg. That is a hint, not a confirmed plot point. The mid-credits cave paintings show Godzilla fighting a Kong-like creature while Titans gather at Skull Island, which links back to Kong: Skull Island and which Godzilla vs. Kong later delivers. In the post-credits scene, Jonah's group buys King Ghidorah's severed left head in Mexico, which leaves the villain side holding a piece of the defeated Titan.
Compared with Godzilla (2014), this ending is the more crowded one, with four named Titans deciding the outcome. It is also the film in the series where the closing image is the Titans' reaction, the bow, rather than only the human survivors.