Nice illustration of the "row picture" of matrix multiplication. The "column picture" is equally important. This intro lecture by Prof. Strang is a really good lesson on that: https://ocw.mit.edu/courses/mathematics/18-06-linear-algebra...

Also, this short tutorial on linear algebra covers the row + column pictures on the first page: https://minireference.com/static/tutorials/linear_algebra_in... (disclaimer: I'm the author)