bash is standard, it’ll always be installed, it has to be there tbh. [I think] zsh is now default on new Mac’s It’s just a slightly different shell, has better ergonomics. They work basically the same. Some people want bash, some people want zsh (or install fish or whatever). Same as how you’ll have at least three text editors preinstalled (vim, emacs, nano) (EDIT four, there’s Pico as well).
It’ll be set to be %, can be anything you want (like mine is set to be dir (the git branch I'm on) > ). Can’t remember which setting it is in your .zshrc/.bash_profile/whatever, you’d need to Google, but you can customise anything basically.