In a market economy, nobody decides how many avocados get grown or who gets to eat them. When demand or supply shifts, the old price no longer clears the market, and the price itself moves until it does. Raise demand or cut supply and watch the gap at the old price push the price to a new equilibrium.
When demand or supply shifts, the shortage or surplus at the old price moves the price, and that change in relative price acts as a signal, an incentive and a rationing device until quantity demanded equals quantity supplied again.