Full spoilers for Godzilla vs Kong below! Even though Warner Bros. did their best to hide him, fans knew that MechaGodzilla was going to be in Godzilla vs. Kong. Unlike many of the other versions of the robo counter part to the King of the monsters this one had unique roots, roots that also ret-con'd a little piece of the previous movie in the MonsterVerse. As fans might recall the post-credit scene of Godzilla: King of the Monsters saw one of King Ghidorah's severed heads rolling up on a beach and somehow it ended up in the hands of Demián Bichir's Walter Simmons and Apex, which is where the new mythos building comes in.
About midway through the film Millie Bobby Brown's Madison, Brian Tyree Henry's Bernie, and Julian Dennison's Josh stumble upon the Ghidorah skull with a ton of wires coming out of. Bernie notes that with the skull they've hardwired the DNA of Monster Zero and turned it into a "living supercomputer." From there though is the new info courtesy of Bernie who notes that Ghidorah's three heads "communicated telepahthically" since their necks were so long and they all had their own brains.
This is an interesting development especially considering that the Ghidorah heads in King of the Monsters all had their own personalities but also seemed to interact with each other visually in the film as well through facial expressions or snapping at each other. For Godzilla vs. Kong's MechaGodzilla they use the Ghidorah brain/skull/nervous system to tie Shun Oguri's Ren Serizawa into the robot and allow him to control it, which goes spectacularly wrong as Ghidorah's consciousness takes over and wails on Godzilla via MechaGodzilla. It's worth noting that the previous versions of MechaGodzilla all mostly have their own origins, some of them alien, and that this is yet another new take on the kaiju.
Director Adam Wingard revealed in an interview his idea for the design of MechaGodzilla came from a surprising place, the live-action Transformers movies.
"I remember watching the third Transformers movie in the theater," Wingard told Inverse. "It was one of those things where I was with a date, and we got there late, and the only seats that were available were literally in the front row. And so I'm sitting there watching Transformers 3 in the front row, and I couldn't tell what the hell was going on... The Transformers, they just looked like metal. They looked like a plane crash. So I thought, that's the thing I wanted to avoid. They were too complex. There are too many moving parts, and I couldn't latch on to anything. Nothing felt iconic with that Transformers design."