That's a great many questions.
It really depends on what you want to emphasize. If it's all about your characters' stories, then soft worldbuilding is generally a good idea. That lets you build the world around your characters so you can make the story work no matter what. If it's more about hard plot, then hard worldbuilding is better. It gives you a clearly-defined set of rules that you can build your story within.
Worldbuilding applies to every aspect of your world: setting, history, magic, etc. The extent you go to is up to you. Hardies generally have a lot of defined details that you can drop into the story whenever you need, while softies are more of a loose thing, where it doesn't really matter what you add and where.
I mean... there're probably just author discussion threads in this subforum somewhere, but otherwise I have an old thread called RightingWrite. I actually follow that one and get notifs from it, so... yeah. Shameless self-plug