A session zero is basically a meeting where the DM and players talk about what kind of campaign they will play, the setting, whether the game will be more combat or RP heavy, whether it will be serious or humorous, topics or themes they’d rather avoid and those they’d like to explore, and maybe a bit about the player characters and how they fit into the story. It’s a good chance to get to know the other players and see whether you’re a good fit for the group.
I’d personally want to support making the player characters during session 0 (maybe not the full character sheet, but at least personality/background and class).
I’ve done campaigns where everyone came into the session 0 with their characters all ready to go and the idea of the characters being adjusted wasn’t really brought up but more the rest of the aspects you mentioned. Differences and some antagonism between characters can make things really interesting, but at least in one of them two of the characters fundamentally wouldn’t ever be in a party together. One of the players felt some cognitive dissonance in wanting their character to stay in the party, but not being able to find a rational reason they would, and kinda had to retcon their character.
