Fill different iPhones screen when using camera view for augmented reality Xcode

Question

I am working on an app which places labels of cities on top of the cameraView for augmented reality. When I open the camera I hide the controls and navigationBar and toolBar. I use the following code I found online to resize the camera view to cover the whole screen. I only have an iPhone 7 in landscape mode. When I use this the height of the screen is 320 (instead of 375 that it should be) and the length is 677. How can I make it so the screen size is the actual size of the phone screen and how do I change it for each different iPhone? I can't use it in simulator to see since you can't use augmented reality in simulator.

picker = [[UIImagePickerController alloc] init];

    picker.allowsEditing = NO;

    picker.sourceType =  UIImagePickerControllerSourceTypeCamera ;
    picker.showsCameraControls = NO;

    self.picker.navigationBarHidden = YES;
    self.picker.toolbarHidden = YES;
    if([[UIDevice currentDevice] userInterfaceIdiom] == UIUserInterfaceIdiomPhone){
        CGSize screenSize = [[UIScreen mainScreen] bounds].size;   // 320 x 568

    float scale = screenSize.width / screenSize.height*5/3;  // screen height divided by the pickerController height ... or:  568 / ( 320*4/3 )

    CGAffineTransform translate=CGAffineTransformMakeTranslation(0,(screenSize.height - screenSize.width*4/3)*0.5);
    CGAffineTransform fullScreen=CGAffineTransformMakeScale(scale, scale);
    picker.cameraViewTransform =CGAffineTransformConcat(fullScreen, translate);

I don't understand the logic behind this and think it is originally for an iPhone SE. If there is a tutorial that explains this or if someone can explain it to me I would really appreciate it. Thanks