Concerning Nightborne, I had questions about that too - see Mahri’s reply here: Unlocking Nightborne - #2 by Mahri-atiesh
Very good info there. Be prepared though, it looks like unlocking Nightborne might legit take 20-30+ hours of gameplay start to finish. I’ve already sunk in 6-7 hours and am not even half way to starting the achievement quests chain(s). I’m not going to invest that much time myself, but to each their own. And good luck! 