How do I open all windows when clicking the icon in Ubuntu dock?
You can change the click-action on Ubuntu Dock icons. Open Terminal and run
gsettings set org.gnome.shell.extensions.dash-to-dock click-action 'minimize'
Now if you click on an application icon with multiple open windows, it will switch between minimising and restoring all windows.
Run in terminal:
gsettings set org.gnome.shell.extensions.dash-to-dock click-action 'minimize-or-previews'
The whole action list is here