Iphone as wireless camera monitor?

I'm an independent filmmaker and have recently helped out at a larger scale film shoot downtown. I noticed that a producer had her iphone out and it had a live monitor feed of what was being shot! When i look it up online people say it isn't possible. But I saw it with my eyeballs, so it is.

I know the new GoPros have a Wi-Fi remote app feature, but i'm not talking about that.

I have a Canon DSLR camera.

How does this work and how can I do this?

As an independent film maker I would have thought you would have been able to ask the producer how it happened.
So, what exactly do you want to do? Watch the same content as the producer was watching on to your iphone or try to get some sort of signal from your canon to your iphone. There are folk here who will probably help you if you give a little more info.