The original Heartless (whish contained Shadows and Darksides) were born of the darkness in peoples hearts. The other variations where created by Ansem's experiments.
i.e. Yellow Operas, Wizards, PowerWilds, etc.
All of the Keyholes are unlocked until the Sora (or whomever has the Keyblade that Sora uses) cpmes along and locks it with his (or her) Keyblade. That is what Sora's Keyblade does. We do not yet know what Mickey's Keyblade does or if there are any others.
It is assumed that the Keyblade that Riku used was destroyed. Ansem's world's door to its heart must have had some sort of doorknob. Sora's didn't and many others didn't either. But Wonderland did. Whether there is a doorknob or not doesn't seem to stop the Heartless from getting to the World's Heart.
Concerning DiZ, Ansem, and Mickey. It is still not confirmed if DiZ really is Ansem's shell. If that is the case, then we still do not know if DiZ retained any of Ansem's memories.
i.e. Yellow Operas, Wizards, PowerWilds, etc.
All of the Keyholes are unlocked until the Sora (or whomever has the Keyblade that Sora uses) cpmes along and locks it with his (or her) Keyblade. That is what Sora's Keyblade does. We do not yet know what Mickey's Keyblade does or if there are any others.
It is assumed that the Keyblade that Riku used was destroyed. Ansem's world's door to its heart must have had some sort of doorknob. Sora's didn't and many others didn't either. But Wonderland did. Whether there is a doorknob or not doesn't seem to stop the Heartless from getting to the World's Heart.
Concerning DiZ, Ansem, and Mickey. It is still not confirmed if DiZ really is Ansem's shell. If that is the case, then we still do not know if DiZ retained any of Ansem's memories.