Hullo and welcome!
Do a tag search on 'secondaries/secondary' and 'couples'. There are no real stats on the poly community, and I am hard-pressed to imagine somesuch ever being compiled, either. That being said, my impression is that most people 'stumble' upon polyamory while in a previously-monogamous relationship that is no longer working.
As to the rest of your questions, check out the Life Stories and Blogs sections to see the myriad ways people negotiate polyamorous partnerships.
Me: bi female in my twenties