Question

How do you know that the product of any two numbers greater than 1 must be a composite number?

Answer 1

Because each of the two numbers is a factor of the product, and since the
product has at least those two factors, it's a composite number by definition.

Answer 2

. . . the definition of a composite number is that it is the product of two or more numbers (not including 1 and the number itself) . . . that's what makes it composite . . . so that's how I know . . .