Kingdom hearts fans tell me if i got it right.?

ok, i have questions about kingdom hearts, ok to start, Master xehanort entered terra, and took Control, and became ansem the wise's apprentice, right, then he became a heartless, and his heartless called himself ansem, and his nobody became xemnas. 1, how did xehanort/terra become a heartless, i thought he was this almighty bad ***. ok, and "ansem" went into riku and took control like he did to terra, and for riku to get sora's memory retruned after what happend in castel oblivion he had to deffet roxas who is sora's nobody and for some reason looks exactly like ven, then bring him back to ansem the wise and namine where they put him in fake twilight town, and downloaded his memory to sora, then sora is back and kill's xehanort/terra's nobody xemnas, then something happens to jimminys notebook and they have to give this note to sora leading to kingdom hearts 3, so what happen to xehanort/terra, is aqua still in the world of darkness, and is ven now roxas or is ven still in sora?