Have you played all the Kingdom Hearts games? Because you seem to have misunderstood things immensely. Here, I will summarize it for you. Forget everything that you think you know.
Kingdom Hearts is the heart of everything essentially. More than a hundred years ago, Kingdom Hearts existed and bathed everything in light. People began to fight over it, which started the Keyblade War. In the Keyblade War, countless people died and were lost to the darkness. Darkness covered everything and Kingdom Hearts was then lost to the darkness. But, light survived in the hearts of children and they rebuilt the world we know today.
Years later, a man named Xehanort finds out about this legend, and becomes obsessed and curious with it. He wants greatly to start another Keyblade War and to get his hands on Kingdom Hearts. He puts his hearts inside of Terra to use him as a new vessel, because his old body was too weak and frail. He then researches the heart and darkness under a scientist known as Ansem the Wise, along with his apprentices. Xehanort turns all of the apprentices into Heartless and lets the darkness consume his heart so that he becomes a Heartless as well. He stole Ansem's name and continued to research the darkness and he began to create an artificial Kingdom Hearts out of the hearts of worlds. He, along with other Heartless, and with the help of Maleficent, went out and destroyed worlds and took their hearts. The artificial Kingdom Hearts he created is in the Realm of Darkness. There is a door that leads to the realm of Darkness called the Door to Darkness and the artificial Kingdom Hearts resides right behind that door. Sora puts a stop to Ansem/Xehanort's Heartless and seals the Door to Darkness along with Mickey, which destroyed the artificial Kingdom Hearts and caused all the hearts to return to the worlds.
Meanwhile, Xehanort's Nobody, Xemnas, has created an organization that consists of the other apprentices of Ansem the Wise's Nobodies and a few other members. When an Emblem Heartless is destroyed by a Keyblade, the captive heart that's inside of the Emblem Heartless is freed. Xemnas and the Organization capture these hearts and create an artificial Kingdom Hearts out of them. Sora and Riku put a stop to him though. The original Ansem the Wise tried to create a machine that could copy the artificial Kingdom Hearts into data, but it failed and the machine malfunctioned and exploded the artificial Kingdom Hearts.
When someone's Heartless and/or Nobody is destroyed, the original person is recompleted, so when Sora destroyed Xehanort's Heartless and Nobody, he has now been recompleted. He has formed a new Organization and now has some sort of plan to get the original Kingdom Hearts.
Also, we currently do not know what the Door to Light is. It's just a mysterious door that opened and freed Sora and Riku from the Dark Margin.