Short answers, pulled from the story.
START I barred both parties from deploying more than 6,000 nuclear warheads and a total of 1,600 intercontinental ballistic missiles and bombers. Its full implementation, completed in late 2001, resulted in the removal of about 80 percent of all strategic nuclear weapons then in existence.
START I was signed on the 31st of July 1991 by US President George H. W. Bush and Soviet General Secretary Mikhail Gorbachev. It entered into force on the 5th of December 1994 and expired on the 5th of December 2009.
US President Ronald Reagan first announced the START proposal in a commencement address at Eureka College, his alma mater, on the 9th of May 1982. He formally presented the plan in Geneva on the 29th of June 1982.
After the Soviet Union dissolved in 1991, the Lisbon Protocol, signed in May 1992 and effective December 1994, bound the successor states Russia, Belarus, Ukraine, and Kazakhstan to the terms of START. Belarus, Kazakhstan, and Ukraine eventually transferred or disposed of all inherited Soviet nuclear weapons.
START I provided twelve different types of inspection, the most demanding verification regime of any arms control agreement at the time. It required data exchanges on all offensive nuclear systems, on-site inspections, continuous perimeter monitoring, and mandatory sharing of telemetry from ballistic missile tests, with encryption of that data banned.
New START, signed in Prague by Barack Obama and Dmitry Medvedev on the 8th of April 2010, replaced START I. It capped deployed warheads at 1,550 and entered into force on the 5th of February 2011. Russia suspended its participation on the 21st of February 2023, and the treaty officially expired on the 5th of February 2026.