A small white window will appear. Move your mouse into that window and press any key. The terminal will flood with output. Press Ctrl+C in the terminal to exit.
sudo apt install xbindkeys # Debian/Ubuntu
In other words, XEV keycodes are the language that your keyboard speaks to your Linux system. By understanding these keycodes, you can unlock a world of customization possibilities, from creating custom keyboard shortcuts to remapping keys to suit your workflow.