Examples are based on Free Desktop standards and demonstrate the most common uses of desktop files, but not all.
User-space .desktop files can override the system wide in the typical way.
/home/alex/.local/share/applications/foo.desktop
Will override
/usr/share/applications/foo.desktop
Therefore we can add additional actions on apps.
/home/alex/.local/share/applications/firefox.desktop
Add some quick links on Firefox
...
Actions=open-google-youtube;open-gmail;
[Desktop Action open-youtube]
Name=YouTube
Exec=firefox https://www.youtube.com/
[Desktop Action open-gmail]
Name=YouTube
Exec=firefox https://www.gmail.com/
...
For quickly creating custom application files, create a desktop template on Gnome templates directory
$HOME/Templates/newDesktop.desktop
Then on
$HOME/.local/share/applications/
Right click in Gnome Files, New Document-> newDesktop and modify it!