How to screen record on my iPhone ?

Pull down Control Centre and tap the icon that looks like a "dot inside of a circle". If you don't see that, you might need to add it.

You can long-press that icon in order to turn on the microphone. With the microphone off, it will still include sounds that your phone produces, such as when playing a video or music, but it won't record your voice.

During recording, a red indicator shows at the top of the screen. Tap that to end the recording. It gives you the option to save the recording to your camera roll.