Question on the use of cameraMatrix input to undistortRectifyMap function in openCV

I am using undistorRectifyMap python3 function and read the openCV documentation however I still am not quite clear on what exactly should be passed as input for this parameter.

I am using iPhone XR and I have calibrated the camera using the sample code provided by openCV and got the out_camera_data.xml file.

Per this openCV tutorial,

camera matrix includes information like focal length (f_x,f_y), optical centers (c_x, c_y) etc. It is also called camera matrix.

Doing a simple google says that the wide-angle lens on an iPhone Xs or Xs Max has a focal length of 4.25 mm. From this how do I get f_x and f_xy? Similarly how do I get c_x and c_y? I don't have the intrinsic parameter details provided in the out_camera_data.xml but it does have extrinsic parameters.

Can someone help me point out a documentation on how to get these details? Thanks for any help!