How do banners work?

I'm starting out again as I haven't played in years and I have lost my account.

However I wanna know how banners work.

For example what if let's say I want to have hu tao for my build, how often will she come out so I can pull for her? Do all the new and old characters get reruns?

I know it's a single player game and it's more involved in the story then having to fight for ranks and leveling up however I do not want to ulock a character like furina even though I still haven't been to Inazuma. I feel like that would be useless since how can I upgrade them.