Short answers, pulled from the story.
The Berlin Wall fell on the 9th of November 1989 when crowds of East Germans pushed through the Bornholmer Straße checkpoint. Guards hesitated and stepped aside, allowing people to climb atop the concrete structure while fireworks exploded over the Brandenburg Gate.
The dissolution of the German Democratic Republic occurred on the 3rd of October 1990. This event concluded an irreversible process that began with the fall of the wall in late 1989.
The Two Plus Four Treaty signed in Moscow on the 12th of September 1990 granted full sovereignty to a reunified German state. Full sovereignty took effect on the 15th of March 1991 after all ratifications were deposited with the German government.
Five new states emerged from former East German territory: Brandenburg, Mecklenburg-Western Pomerania, Saxony, Saxony-Anhalt, and Thuringia. These regions held their first elections on the 14th of October 1990.
West German Interior Minister Wolfgang Schäuble signed the Unification Treaty alongside East German State Secretary Günther Krause on the 31st of August 1990. This document established the legal mechanism by which East Germany would join the Federal Republic of Germany.