What is primordial soup and how does it relate to the Big Bang Theory?
There is no direct relationship but they both relate to the question of how order can be created out of chaos.
The Big Bang is a theory of the origin of the universe. The Big Bang is the idea that there was a superdense ball of matter that exploded.
Out of the chaos of the explosion the order of the universe came into being.
The primordial soup is a theory of the origin if the first cells. The theory is that inorganic molecules became concentrated in a closed warm pond of water. Then out of the chaos and disorder of the random molecules the first cell was formed.
Generally the second law of thermodynamics says that the universe is moving from order to disorder. Both the theory of the primordial soup and the theory of the Big Bang contradict this law of chemistry and physics.
Also both of the theories are based on an assumption of material realism. The assumption is that everything happens by uniform natural causes of matter and energy. That is matter and energy are all that exists.