What is the action (command) to show all open windows in Debian?

The default action to show present windows is

ctrl+F9

This will zoom out and show all open windows.

Alternatively

If you go to

System settings - Desktop behavior - Screen edges

You can set

present windows (all desktops/current desktop/current application)

On one of the 8 screen edge actions, that way you just push your mouse cursor to whichever edge you created the action for, and it will accomplish the same thing.