Short answers, pulled from the story.
Georgy Flyorov sent his first classified letter to Soviet leader Joseph Stalin in April 1942. This letter warned that Western scientists had stopped publishing on nuclear science because they were secretly building atomic weapons.
Stalin authorized engineering physicist Anatoly Alexandrov and nuclear physicist Igor Kurchatov to lead the effort. Laboratory No. 2 near Moscow became the new home for this research under their direction.
Richard Rhodes noted that Kurchatov learned enough through espionage to transform the Soviet program and accelerate it by a full two years. American communist sympathizers controlled by Russian officials shared classified information that greatly aided the speed of the Soviet nuclear program.
The Soviet Union secretly conducted its first weapon test called RDS-1 on the 29th of August 1949 at Semipalatinsk Test Site in Kazakhstan. The energy yield measurement and design mostly followed American Fat Man using TNT hexogen implosion lens design.
The largest thermonuclear weapon ever detonated was Tsar Bomba on the 30th of October 1961 with an estimated yield between 50 and 57.23 megatons by different sources. Today all Russian sources use 50 megatons as the official figure for this explosion.