Generally everything that is not character building, but the two overlap as well.
But for examples, the geography, the different political factions, DFs, Haki, the history of the world, details like Den Den Mushi and Vivre Cards, bounties, even fashion, that’s all world building.
Character building is fleshing out characters, their likes and dislikes, their appearance, why they do the things they do, their personality, their skills. People seem often to think that character building just consists of a character making a big change and that’s not true.
Then in some cases they interact. Robin being a major one. The reason hee character is the way it is is because of the world.
She lived in a world where their was a period of history hidden by the ruling government
Her parents researched this hidden period
There are things called DFs that give strange powers
Robin ate one
This combined leads to Robin being an lonely outcast to most of the island she kives, as she is considered a freak because of eating a DF, plus her family are gone. The only ones she likes are archeologists like her parents, so she becomes an archeologist, and that leads to etc etc.