Since there's no "Screenshot" app in macOS 10.13 or earlier, user should use the "Grab" app to take screenshots and the QuickTime player to start a screen recording

To record a screen in QuickTime, you need to go by following sequence:

1) Open QuickTime.app from Utilities folder in Launchpad

2) Go to File > New Screen Recording... and you'll see a following window

In the dropdown near the white-red circle, you'll be able to select the audio input method. Also, you can say there if you want mouse clicks to be visualised by the system, so the video will feature following elements:

-> on click, the black circle will appear

-> on click and drag, the video will show a green circle of the same size

After clicking the white-red button, select the screen portion you want to record and click anywhere on the screen to start recording.