You must plug your Chromebook into the projector in order to set it to mirror.  By default, it will currently (as of 11/2020) always use a new projector or display as a second monitor and not mirror your screen.  For most people, this is not ideal.  To change it:

  1. At the bottom right, select the time.
  2. Select Settings .
  3. In the "Device" section, select Displays.
  4. Select Mirror Built-in Display. This option will appear when your monitor is connected.
Tip: To view your Chromebook screen only on your monitor, press and hold the Decrease brightness  button


Adapted from: https://support.google.com/chromebook/answer/1060909?hl=en