In every Monsterverse movie, there is always some lore that we see get redacted during either the opening or ending credits sequence. This trend all started with Godzilla (2014) and then stayed consistent with every movie including Godzilla vs Kong.
In Godzilla: King of the Monsters, we learned about ancient civilizations that co-existed with the monsters to the point where they were worshipped like gods. Suddenly, these ancient civilizations crumbled as they decided to use the monsters for war creating what the films have called the Titan War. After the war, any monsters that were still alive went down into the Hollow Earth to hibernate and humanity was left to pick up the pieces afterwards. Over time, the stories of these gods were getting more skewed and vague until they were forgotten.
So what kind of lore does Godzilla vs Kong give us this time? Well, not a whole lot outside of Godzilla's and Kong's species were natural enemies.