In a socialist state people decide what they want to do collectively. So most of the time you choose what job you want to do. While the USSR did have conscription for the military besides that people were free to choose any job they had the qualifications for and university and training were free for everyone so that those qualifications did not create unjust inequity. Through consultation with workers across the Union and by having Marxist economists and mathematicians use the best formulas (that often were much more advanced then those developed by Western economists) the Gosplan would create five year plans to set targets on how much of this product or that product each region should make. Then at each individual workplace the workers in their unions would discuss how best to implement the plan given their local material conditions