how to use dual screen ?

The way it works depends on how you set it up.

By default the display is extended so the two screens act kind of like one large screen. Where you can move the mouse and drag programs between them.

Just like with a single screen the apps should remember where they were last placed and so will open on the screen where they were last.

Programs have focus in a dual screen setup in the same way they do in a single screen setup, whichever app you last used will usually have focus.