Short answers, pulled from the story.
Article 76 defined a Union Republic as a sovereign Soviet socialist state that united with other republics to form the Union of Soviet Socialist Republics. This status created a paradox where sovereignty existed on paper but was limited by membership in the Union.
The number of union republics stabilized at fifteen from 1956 until dissolution in 1991. The Karelo-Finnish Soviet Socialist Republic was the only union republic to be deprived of its status later when it was downgraded to an autonomous republic within the Russian SFSR in 1956.
Ukraine and Byelorussia joined the United Nations General Assembly as founding members in 1945 alongside the USSR itself. These changes permitted Republic-level commissariats for foreign affairs and defense, allowing them to be recognized as de jure independent states in international law.
Lithuania, Latvia, and Estonia assert that their incorporation into the Soviet Union in 1940 under the provisions of the 1939 Molotov Ribbentrop Pact was illegal. Their position holds that these nations remained independent countries under Soviet occupation according to their stance supported by the European Court of Human Rights cases on Occupation of Baltic States.
On the 8th of December 1991 remaining leaders of the republics signed the Belavezha Accords which agreed that the USSR would be dissolved. President Gorbachev announced his resignation on the 25th of December and turned all executive powers over to Yeltsin while the Council of Republics voted to dissolve the Union the next day.