iOS 11 comes with a built-in screen recorder. There is no app for this functon, instead it is hidden in the Control Center.
In order to use the screen recording function, you need to add its button into the Control Center. Go to Settings app > Control Center > Customize Control. Scroll down to bottom section and tap the “+” button for “Screen Recording”. This will add the screen recording button to Control Center.
To start screen recording, activate Control Center and tap on the “Screen Recording” button. After a timer of 3 seconds, the recording of your screen activities will start. When iOS is actively recording what you do on your iPhone, the status bar will be red in color.
To stop the recording, tap on the red status bar and select “Stop” when prompted. Alternatively you can tap the same “Screen Recording” button in Control Center to stop the recording. The screen recording is saved as video into iOS Camera Roll.