The Russians had no reserves, the Red Army had 10,000,000 men killed or captured during the war. The Soviets drafted men and women from 16 to 30 years of age during the war. Red Army troops got minimal training, in theory ninety days, much of which was wasted since weapons and ammo were lacking in training units, but often that was shortened to as little as three weeks with raw recruits being fed directly into units in combat for “training”. By 1944 the draftees had been living on a thousand calories a day for three years. Russian civilians were on starvation levels of rations for the entire war. What food there was went to the Red Army.
Medical care was poor, units were left in combat until they took over sixty percent casualties, then pulled back and consolidated with the survivors of other units and green recruits and fed right back into combat.
by the Battle of Berlin, the Red Army was on its last legs.