When your game is being targeted at those platforms that permit multi-touch, you should be using these functions. They work the same way a PC would work if it had multiple mice attached and the constants that are being checked for are the same as those used for the standard mouse functions:

The functions can all take a "touch" value that corresponds to a touch on the device display, however the maximum number of touches that can be detected will depend very much on the device and the OS being run. For example, some low end Android devices may only be able to detect 2 or 3 touches at the same time, while a UWP device may be able to detect up to 10. Note too that on touch screen devices, a double tap is returned as the equivalent of a right mouse button event. So, if you need to detect multiple taps in quick succession, for example, you will need to check for both the mb_left and mb_right. This functionality can be toggled on or off using device_mouse_dbclick_enable.

Below you can find a list of all the device specific control functions:

1. device_mouse_check_button
   
2. device_mouse_check_button_pressed
   
3. device_mouse_check_button_released
   
4. device_mouse_dbclick_enable
   
5. device_mouse_x
   
6. device_mouse_y
   
7. device_mouse_raw_x
   
8. device_mouse_raw_y
   
9. device_mouse_x_to_gui
   
10. device_mouse_y_to_gui
    
11. device_is_keypad_open
    
12. device_get_tilt_x
    
13. device_get_tilt_y
    
14. device_get_tilt_z
    

Please note that on mobile devices (Android, iOS and Windows Phone), you can use the back button at any time, as it is mapped to the keyboard backspace which is the GameMaker Studio 2 keyboard constant vk_backspace, so you can do checks for that key and perform actions on those devices as if the back button had been used.