@berak thank you so much.
I am not sure from where, but lets sum up:
OpenCV uses fx with a pixel on the image x and the optical canter cx.
All of them in pixel units, while doing the same with fy, y, and cy in order do the calculations, like for example minimizing the reprojection errors of the different solvePnP methods.
OpenCV is placing the image plane at which distance form the camera origin? A distance fx (in pixels this would be confusing) for the calculations on x axis, and at a distance fy for the calculations on y axis?
I am also finding other sources that mention they normalize it to a distance f = 1.