Short answers, pulled from the story.
The Polish-Lithuanian Commonwealth was officially created on the 1st of July 1569 when deputies from Poland and Lithuania signed the Union of Lublin in Lublin. This document merged two separate states into a single federative entity known as the Polish, Lithuanian Commonwealth.
Mikołaj Radziwiłł the Red and his cousin Mikołaj Radziwiłł the Black vocally opposed the terms of the Union of Lublin before leaving the assembly in protest. They feared the loss of Lithuanian autonomy but were forced back to negotiations by Sigismund II Augustus using his authority to enforce the Act of Union contumaciam.
After the death of Sigismund II Augustus in 1572, the hereditary line ended and triggered an elective monarchy system where kings reigned but did not govern. The Sejm held legislative power alongside a Senate, and individual deputies possessed the right to use liberum veto to nullify all legislation passed during a session.
By 1618, territorial expansion brought the total number of people in the Polish-Lithuanian Commonwealth to 12 million. At that time nobility made up 10% of the entire population while burghers comprised around 15%.
The Polish-Lithuanian Commonwealth had been completely erased from the map of Europe by 1795 after three partitions. Poland and Lithuania remained without independent status until 1918 when they were re-established as sovereign nations.