Streaming how do you stream on an iPhone?

The source is where the streaming video is coming from -- YouTube, HBO, Disney+, Hulu, Netflix, Amazon, BBC, Los Angeles Philharmonic, etc. Most of these play in any web browser, Safari for example. You just go to the relevant website, locate the video and play it (although some sites may require a sign-in, a fee or a subscription).

There are also some places that have their own dedicated player app.

When you said you wanted to "see a concert live" I thought you meant "watch a live concert."

But if you meant that you wanted to use your iPhone to stream audio & video for other people to watch over the internet, then you need a live streaming app on your iPhone. There are a number of those apps in the App Store. I don't use them myself, so there's not one I could recommend.

The answer depends on the source you are trying to stream from. What are you trying to stream from?