How do people blur out certain parts of a photo on the iphone?

For like instgram and stuff

It is supposed to emulate a low depth of field (only a few things in focus).

This makes things look like they were either (a) shot with a fast professional camera lens which only makes the subject fully focused or (b), makes the objects look smaller, because macro lenses also have shallow depths of field.

But because the parts of the photo are blurred regardless of how close they are, and they are blurred artificially, it doesn't look as good.

Professional and skilled amateur photographers often spend lots of money on cameras and lenses and lots of time on technique to get out of focus parts to look just right. The quality of the out of focus elements is known as bokeh.

They use free apps like Pixlromatic and Xiamen Meitu to edit for the bokeh effect. I don't use those for my photos so I don't know how it's done. Install them and try it out.