They don't rebirth because they're good enough to fight and defeat someone without having to have the extra stats from rebirthing. It's more of a disrespectful thing towards their opponent. “If I defeat you with my Pure when you're a Rebirth, then you're trash.”
You can unlock them from beating raids (Broly or Cell), by buying them from the Raid Crystal Shop, buying gamepasses (currently only Goku Black), or past/future events.
Someone told me to rebirth before prestiging because you only get one and it adds 300+ to base and when you prestige you get more out of the base stat cause of the 300+. But I guess not :/
I would say it all depends on how many time you plan on prestiging. You will lose the 300 stats given from rebirthing if you don't prestige at a very high level, and you can lose it by prestiging multiple times.
You should be able to change it at any level. Most fighting styles are attainable from gamepasses or from NPCs. Such as Goku Black NPC (cost 1 million Zeni). But if you're saying you don't know where to change your fighting style, go to the Menu, Inventory, then Weapons. You should see all the fighting styles you currently have there.
You must fight all bosses on Earth that are able to be fought with your level. Then once you level 80, you must fight Bio-Broly. Until you're level 100. He’ll be difficult but it's possible.
