Simple - this is the basic autofire mode. When you point the crosshair at an enemy, your weapon will fire. However, the sniper rifle and rocket launchers have manual fire and don’t have the autofire.
One-tap ADS - this mode disables the autofire and gives you a fire button. When you fire, you automatically aim down the sights for better accuracy as long as you hold the button down.
Hip-fire - a classic manual fire mode. You have the fire button and will shoot from the hip by default, but also have the option to double-tap the screen to aim down sights.
Per Weapon - the "Per weapon" mode is currently not in its final state. For now, it works mostly as "Simple" - the difference is with how the sniper rifle and rocket launcher are controlled:
In "Simple", you shoot wit these weapons in hip fire and double tap for ads.
In "Per Weapon" these weapons use one-tap ads instead.