In terms of what you see on it, your pen display is just another monitor/display to your computer. So, you can control how the computer shows its desktop to the pen display.
You have two options:
Extend the desktop - this means you can see different things on the pen display from your monitor. This is what I recommend you use.
Duplicate the desktop (aka "Mirror ther desktop") - This means your pen display and monitor will try to show the same exact thing. Sometimes this is very useful but generally I think Extend is better choice for most people and situations.
The steps depend on which operating system you are using.
Open Display Settings
The displays your computer is connected to will be shown and each one is numbered. The one in blue is the display currently selected.
If a display has two numbers, that means the desktop is being duplicated across those displays
NOTE: If you only see one display then the these settings do not apply.
Select the display that corresponds to your tablet
Towards the button right (to the right of the Identify button) you will see a dropdown setting that controls how the desktop is being applied to this display. Depending on how the display is already configured, the dropdown will say either
Extend desktop to this display
Duplicate desktop on X and Y
X and Y will be the numbers referring to the displays
Switch the dropdown between the Extend or Duplicate option as needed.
Go to Displays > Display Settings
Select your monitor, then set Use As to Primary Display
Then select your drawing tablet's display and set Use As to Mirror.