Where is the battery percentage?

That is correct, he never said there was such a button. The easiest way to check is pull down Control Center or use the widget.

Go toSettings, scroll down to Battery. Click on Battery for all details of battery.

You can also see it by swiping down from the upper right corner of the screen to bring down Control Center. The battery's charge percentage will be displayed there. You can also use a battery Widget when you swipe all the way to the right to expose the Today screen.