Short answers, pulled from the story.
The first nuclear weapon detonation occurred on the 16th of July 1945 at the Trinity Test Site in New Mexico. J. Robert Oppenheimer, the scientific director of the Manhattan Project, witnessed the event and later recalled a verse from the Hindu scripture Bhagavad Gita.
The United States Army Air Forces detonated the uranium gun-type fission bomb Little Boy over Hiroshima on the 6th of August 1945. Three days later on the 9th of August, the plutonium implosion-type fission bomb Fat Man was detonated over Nagasaki, causing injuries that resulted in the deaths of approximately 200,000 civilians and military personnel.
Harry Daghlian died 25 days later on the 15th of September 1945 and Louis Slotin died nine days later on the 30th of May 1946. Both physicists conducted experiments on a plutonium-gallium core at Los Alamos National Laboratory between the 21st of August 1945 and the 21st of May 1946 when human error caused the core to enter prompt criticality.
The largest nuclear weapon ever detonated was the Tsar Bomba of the USSR on the 30th of October 1961. This three-stage weapon released an energy equivalent of over 50 megatons.
The United States, the Soviet Union succeeded as a nuclear power by Russia, the United Kingdom, France, China, India, Pakistan, and North Korea are the only countries known to have detonated nuclear weapons and acknowledge possessing them. The total from all stockpiles peaked at over 64,000 weapons in 1986.
The Doomsday Clock was moved to 89 Seconds to midnight in 2025 following the escalation of nuclear threats during the Russo-Ukrainian war. The clock previously reached 90 seconds in 2023, which was the highest likelihood of global catastrophe since the existence of the Doomsday Clock.