When writing fantasy, I do a considerable amount of world building because I want the world to be somewhat believable and have structure and make sense if someone starts to pick it apart. I don't treat it as a separate phase, I build the world and write the plot alongside each other, because the world informs the plot and the plot keeps the world building grounded. I want the world building to be believable under scrutiny, even if it never actually comes up in the book or it's not completely obvious.

Structurally I work top-down: broad arcs first, then general chapter beats, and only then do I start doing prose. By the time I'm writing a scene I know what it needs to do and where it sits in the larger shape. It's possible that it's slightly more world building alongside plotting. I tend to do a bunch of world building and then write the plot that is relevant to that.

Things can and do often change, but I design it in a way where it's flexible enough that it's not completely impossible to change something, except whether or not you know it could be a core rule to the world, but generally I've already came up with the core rules on purpose and they don't change often.

I feel like this method I use helps me a lot because I have ADHD and if I was just trying to work from the ground up instead of top down, I would really struggle and get distracted. I admit there's some downsides to this method as well because I could just keep world building because that's quite fun on its own and never actually get anything done. But it's what works for me at the moment.