How did some of the panels of the baptistry door appear in the Opera del Duomo?

I suppose you are talking about Florence without saying so. The last set of panels for the Baptistry doors, the Doors of Paradise, are the most valuable ones. They have recently been exchanged for copies and the originals are displayed in the museum "Opera del Duomo", where they are well guarded. An extra bonus is that visitors can watch them at close range there.