Q1: why did  Arwen join bts when she already had debuted?

A: well, Arwen was going through a pace of different things. Good and bad. And she needed someone to support her other than she already gained fans. Bts needed another vocal, a vocal with a silky voice and a lovely face. A member that would gain attention and be caring and love. Well, Arwen was exactly what bts needed.

Q2: How did BigHit decide to add Arwen?

A: well, she had just posted her newest cover or collaboration with Alan walker. Unity. It was getting millions upon millions of likes and views. This was not so important for the young idol. She just loved to sing. She had it difficult, as I already said in the other question. When bang pd called her, she was overwhelmed to join one of the becoming most prominent groups in k-pop. She joined the next week, and ARMY'S went wild when they saw her for the first time.

Of course, some fans doubted her and said she wasn't fit to be in a group of boys. Of course, this mars her second guess it all, but her new members and staff escorted her, helping her become a better version of herself, and for that, she is always thankful.

I think that's all I can answer; for now, ask any more questions, and I'll answer them in the following Q&A:)