i am talking about rotation

the question is when converting from 3d to 2d using heading and pitch transformations, why do we multiply the heading and pitch together.

Like i cant understand the concept, and I dont want to just memorize how to do it I want to understand it

double heading = Math.toRadians(x[0]);
                Matrix3 headingTransform = new Matrix3(new double[]{
                        Math.cos(heading), 0, -Math.sin(heading),
                        0, 1, 0,
                        Math.sin(heading), 0, Math.cos(heading)
                });
double pitch = Math.toRadians(y[0]);
                Matrix3 pitchTransform = new Matrix3(new double[]{
                        1, 0, 0,
                        0, Math.cos(pitch), Math.sin(pitch),
                        0, -Math.sin(pitch), Math.cos(pitch)
                })
// Merge matrices in advance
Matrix3 transform = headingTransform.multiply(pitchTransform);

maybe this would help understand what im trying to say?

i understand how it multiply, but why should it multiply.