https://www.gamespot.com/articles/fortnite-mobile-season-5-update-introduces-autofir/1100-6460356/

Fortnite's Season 5 officially launched today, bringing with it new challenges (and rewards), a whole new area of the map with the World Collide event, Rifts to send players skywards, and Karts to transport your whole team. Along with all the new additions are a few tweaks and changes on the game's mobile version, including the addition of autofire.

Autofire, which enables the player's weapon to automatically fire when the aim is over an enemy in range, can now be enabled. This new option should make the touchscreen controls easier to manage, as you previously had to swipe to aim and tap to fire with the same thumb. Players can activate autofire after launching by going to Options, then Custom HUD Layout, then Select Fire Mode. There are now three options: Tap-to-Fire, Dedicated Fire Button, and Autofire.