Repressenting relations using matrices

Hi everyone!

I am struggling a bit with a question, it is pretty basic, and I have read through this part in the book, I just don't quite understand how this works.

Question:

Suppose that A = {1, 2, 3} and B = {1, 2}. Let R be the relation from A to B containing (a, b) if a is inside of A, b is inside of B, and a > b. What is the matrix representing R if a₁ = 0 , a₂ = 2, and a₃ = 3 and b₁ = 1 and b₂ = 2?

In the book they do give a matrice as a solution, but they don't explain how they got the matrice. So, I was hoping someone could explain to me how to think about this.

Thank you all for reading!

I guess Mij is 1 if and only if a_i is in relation with b_j and 0 otherwise. (M is the matrix)