Thanks. Here's an example/ one of my practice cases:
I multiply the X, Y, Z and W component with the first colum of the 4x4 matrix.
For Y I multiply the XYZW components with the 2nd colum of the matrix, etc.
So the resulting new X component, is determined by a multiplicaton of all 'original' vector components with the 1st column of the matrix.
I'm not 100% sure, but I believe directx/3d uses/ expects 'row major' matrices.
And to get for example the seperate axis's, I take the 1st three values of the 1st row for X, 2nd row for Y, 3rd row for Z (containing the rotations/scaling).
This example is a rotation matrix with X rotation is 90%:
Where I believe the 1st 3 values of the 1st row, define a vector which I use as 'X axis' (my explanation in dummy: X and Z influence the result, because there's rotation around Y).