Short answers, pulled from the story.
The phrase papal bull emerged around the end of the 13th century for internal administrative use within the Vatican. It became an official title by the 15th century when the Apostolic Chancery established a register called registrum bullarum.
Papyrus remained the standard writing material until the early years of the eleventh century. A rough kind of parchment rapidly superseded papyrus after that transition point.
Benvenuto Cellini received 50 scudi in 1535 to recreate the matrix for Pope Paul III's seals. He carved the apostle faces with greater attention to detail than previous examples and added fleurs-de-lis heraldic devices belonging to the Farnese family.
The term bulla derives from Latin bullire meaning to boil since melted material softened for impression. This process describes how the metal seal was formed to create the document's authentication.
Red ink stamps replaced lead bullae starting in the late 18th century. Very formal letters like the bull convoking the Second Vatican Council still receive the leaden seal today.