When I first started playing ttrpgs, I remember we would write pages of character backstory to figure out who our characters were Like the story had already happened to them! This didn’t work out because that story wasn’t relevant to the new story It never helped us develop our characters

I have the exact different opinions and experience as far as long character backstories, of course that is VERY setting and character dependant on when long backstories work out well! But ttrpgs are certainly not one size fits all, varied experiences for varied tastes ^^

I always like to ask my players: When is a character's past usually told in a novel? At the very beginning in an infodump when we don't even know who this character is - or in bits and pieces over time when the reveal is relevant to the story?

Yes, this. A friend of mine used to write really epic backstories, and another friend said that it didn't make sense to do that, "because the most interesting thing in their lives have already happened to them, start and finish".

As a bonus, I really like games where it facilitates mechanical flashbacks into your characters history Why is this scene in the present significant to a character because of something in their past? Let’s find out! Now it’s true! These are hard to concoct on the fly without mechanical support

Yeah this was a HUGE trap for me and my groups when we started playing. Exactly as you describe!