Where did World War 1 mainly take place in?

Europe. The Eastern Front was on the Austria-Hungary/Russian battlefield, whereas the Western Front was on the German/France/Britain battlefield.

Although, as the name suggests, it took place in all parts of the globe, on land and sea, the focus of the fighting for the main combatants was the trenches of Western Europe, with other actions occurring in or around Russia, Italy, Salonika, Gallipoli, Africa, and Arabia.