The law of supply, the law of demand, and the concept of equilibrium price are covered in the first chapters of any economics textbook.
Finance draws primarily from economics and accounting.
It's rare for a market to be in equilibrium. Classical economists didn't worry about this because they believed equilibrium would be reached sooner or later.
The interplay of supply and demand determines how much is sold, who sells, who doesn't sell, and who cannot access the good or service.