Even looking at the definition used to make me sweat, let alone trying to comprehend the pattern. Yet, there is a stunningly simple explanation behind it.
Let's pull back the curtain!
First, the raw definition.
This is how the product of A and B is given. Not the easiest (or most pleasant) to look at.
We are going to unwrap this.
Here is a quick visualization before the technical details.
The element in the i-th row and j-th column of AB is the dot product of A's i-th row and B's j-th column.