What keyboard shortcut do you use for Extended display in Ubuntu mate 20.04?
Welcome to the Ubuntu MATE Community!
I'm not aware of a "standard" key combination or shortcut to extend the screen on any Linux distribution. As an Ubuntu MATE user, I know that if you use the Displays application in the Control Center to extend the screen, your computer will remember the settings between reboots. Here is the help topic on Displays:
I've found that Ubuntu MATE also remembers your settings whenever you plug and unplug the cable to your monitor or even just turn it on and off. Once you've set it, you don't really need a shortcut key unless...
If you have a laptop and you connect to different external monitors, like when you are making a presentation on a monitor or projector you may want a shortcut. I have several laptops, each of which comes with a key that is meant to be pressed to extend or change the screen. I have written a script that extends the desktop and assigned the script to the F8 key on my XPS13. For the XPS13, that's the key that is labeled for this purpose. I have another computer where it's the F9 key they use for this. It just depends on the computer manufacturer. In reality, once you have a script that makes the switch for you, you can use the Keyboard Shortcuts app to assign the script to any key or key combination you want.
The help article on keyboard shortcuts is here: Keyboard Shortcuts
You can use the xrandr command in your script to switch the external display on and off. You will need to connect the display to your computer, then use this article to determine the name, the resolution, and the position of the display that you want to include in the script.
I wrote this article with more detail, if you need it. Create a Keyboard Shortcut To Switch Displays.