Short answers, pulled from the story.
The 1944 Bulgarian coup d'état was the overthrow of the Kingdom of Bulgaria's government on the night of the 9th of September 1944. It was organised by the Fatherland Front coalition, led by the Bulgarian Communists, and carried out by pro-Fatherland Front army units and the partisan People's Liberation Insurgent Army. The coup replaced Prime Minister Konstantin Muraviev's government with a Fatherland Front cabinet led by Kimon Georgiev.
The coup was organised by the Bulgarian Workers' Party (the Communists) and the broader Fatherland Front coalition. War Minister General Ivan Marinov and Zveno-affiliated officers seized key Sofia installations in the early hours of the 9th of September. Kimon Georgiev, leader of the Zveno group, was proclaimed Prime Minister, while NOVA partisan commander Dobri Terpeshev ordered all guerrilla units to occupy towns and villages across the country.
The Soviet Union declared war on Bulgaria on the 5th of September 1944 because the Muraviev government, despite declaring neutrality and ordering German troops to withdraw, had not dissolved the formal alliance with Nazi Germany and had made no moves to normalise relations with Moscow. Partisan actions by Fatherland Front forces continued throughout this period, which deepened Soviet distrust of the new government.
The three royal regents, Prince Kiril, Bogdan Filov, and Nikola Mihov, were arrested on the 9th of September 1944 along with former Prime Minister Konstantin Muraviev. All three regents were executed on the 1st of February 1945. Tsar Simeon II, who was seven years old at the time, remained as a nominal monarch until Bulgaria was declared a People's Republic on the 15th of September 1946.
On the 10th of September 1944, the day after the coup, 8,130 political prisoners were released from Bulgarian prisons. The concentration camps of the former regime, including Gonda voda, Krasto pole, and Lebane, were closed on the same day, and the old Interior Ministry police were abolished and replaced with a People's Militia drawn from former partisans.
The People's Court (Naroden sad) was a special tribunal established by the Kimon Georgiev government in December 1944 to try fascist officials and wartime collaborators. Hearings began in early 1945. Between 4,000 and 30,000 people were killed or went missing in the first four months after the Communist takeover, combining both the court's proceedings and extrajudicial killings by security forces.