When talking about Fortnite, we are referring to one game in three modes.
There’s Fortnite: Save the World (released 2017) for PC and some consoles, Fortnite Battle Royale (released 2018) for Nintendo Switch, iOS and Android and, Fortnite Creative (released 2018) which is also for the Switch and mobile. All three of these are made by Epic Games combining pop culture and the tried-and-tested shooter gameplay that appeals to a wide range of gamers.
Gameplay is pretty much identical in the three versions: either you point-and-shoot using weapons found in the playing field, or defend yourself and the objective by creating all sorts of fortifications to shoot safely from.
Popularity-wise, the one that stands out of the three is Fortnite Battle Royale. Drawing more than 12 million players in less than a year, it is no doubt the biggest moneymaker and has also become a culture of its own.