Well, it's quite complicated and I hope i explain it corectly.
At some Point of the story, Ansem SoD got the abillity to travel trough time, but only to a moment of time where a version of himself (his heart) already existed. By doing so, he'd lost his body and only existed as an heart (guy in brown coat). He traveld back to his former younger version; Young Xehanort. Ansem thought him this abillity. So YX was able to visit future versions of himself (Xemnas, Ansem SoD, Vessels who contain Xehanorts heart) and brang them into the future. Meanwhile, Ansem stayed at Deytiny Islands and waited for the events of the original Kingdom Hearts game.
Well, it's quite complicated and I hope i explain it corectly. At some Point of the story, Ansem SoD got the abillity to travel trough time, but only to a moment of time where a version of himself (his heart) already existed. By doing so, he'd lost his body and only existed as an heart (guy in brown coat). He traveld back to his former younger version; Young Xehanort. Ansem thought him this abillity. So YX was able to visit future versions of himself (Xemnas, Ansem SoD, Vessels who contain Xehanorts heart) and brang them into the future. Meanwhile, Ansem stayed at Deytiny Islands and waited for the events of the original Kingdom Hearts game.