Why is the demand for labor called a derived demand?

In the labor market, what are the firm’s demand curve for labor and the workers’ supply curve of labor?

How is a firm’s wage normally determined in the labor market?

How could Amazon decide to raise its minimum wage to $15 per hour, despite the federal minimum wage being fixed at $7.25 per hour?

What are positive and negative effects of Amazon raising its minimum wage to $15 per hour on its employees, total revenue, and other companies and their employees?