Short answers, pulled from the story.
Soviet foreign minister Vyacheslav Molotov informed Japanese ambassador Naotake Satō that the Soviet Union had declared war on Japan at 5 p.m. Moscow time on the 8th of August 1945. The declaration took effect from the 9th of August, marking the end of almost six years of peace between the two nations.
The Far East Command under Marshal of the Soviet Union Aleksandr Vasilevsky planned a massive pincer movement to conquer all of Manchuria. The Transbaikal Front under Marshal Rodion Malinovsky aimed to secure Mukden while the 1st Far Eastern Front under Marshal Kirill Meretskov targeted Mudanjiang, Jilin, Changchun, and Harbin to close the double envelopment.
Most heavy equipment and best military units transferred to the Pacific Theater over three years left the Kwantung Army with obsolete tanks and limited infantry capabilities. Only six divisions existed prior to January 1945, and the force contained many raw recruits and conscripts when the Soviets launched their simultaneous attack on all three fronts.
Soviet soldiers killed and raped Japanese civilians including the Gegenmiao massacre where over one thousand women and children died. Wounded Japanese soldiers often were left to die as the army retreated while looting and terrorizing local people occurred in cities like Mukden and Harbin.
Soviet forces captured scientists from the Kwantung Army's Unit 731 biological and chemical warfare division and sentenced them in the 1949 Khabarovsk war crimes trials. The Soviet Union leveraged this expertise to advance its own military scientific endeavors while punishing those responsible for atrocities committed during the war.