black bars above/below or on sides of photos

You can make it fill the screen, though.

This is a picture that's the size of the camera's sensor, 4:3

Here's the picture that I cropped to 16:9

I cut off part of the truck so that the picture would be just the right size for my monitor and for my TV screen.

If I pinch to zoom in a little bit, then it will cover the whole iPhone screen,

But to do that, I had to cut off the bottom that had the Antelope's feet and shadow, and I lost a bit of the sky at the top.

You can use Edit>Crop to cut off whatever part you like to fit whichever screen you want to fit!

You get borders when the aspect ratio of the photograph is different to the aspect ratio of the screen. Otherwise the photo would bleed off screen.