Short answers, pulled from the story.
Collective leadership is a system that distributes power across a leadership body rather than concentrating it in one person. A single officer, such as a general secretary or party chair, is meant to serve only as primus inter pares, first among equals, within that body.
Collective leadership in the Chinese Communist Party is generally considered to have begun with Deng Xiaoping in the late 1970s, during the reform and opening up period. Official post-Mao historiography holds that Mao Zedong had wrongly abandoned the principle in 1966.
Some political analysts date the decline of collective leadership in China to Xi Jinping's abolition of term limits in 2018. Brookings Institution fellow Jonathan A. Czin argues Xi came to see collective leadership as the source of many of the party's problems after watching Hu Jintao's strict adherence to it prevent decisive action against corruption and inequality.
The Soviet Union, under the Communist Party of the Soviet Union, distributed power among the Politburo, the Central Committee, and the Council of Ministers to prevent one-man dominance like Joseph Stalin's. Vladimir Lenin was held up in Soviet literature as the model leader who ruled in favor of the collective.
Nikita Khrushchev was ousted in 1964 and replaced by Leonid Brezhnev as General Secretary and Alexei Kosygin as Premier. Collective leadership then strengthened through the Brezhnev years and the later leaderships of Yuri Andropov and Konstantin Chernenko.
Parties including Alliance 90/The Greens, Québec solidaire, the Green Party of England and Wales, the Green Party of Canada, the Scottish Green Party, the Green Party of the United States, Left Bloc in Portugal, The Left in Germany, and several Irish parties all practice forms of collective leadership. Many green parties justify this through their movement's emphasis on consensus decision making and gender equality.