Waveshare maintain a wiki page for the Alphabot2.
In the resources section on that page, there are some links to demo code which contains definitions for the allocation of GPIO pins.
It looks like the specific file you want is AlphaBot2-Demo.7z
The compressed file contains a folder named Python:
I've taken the mappings below from these files. Note that this list of GPIO mappings may not be exhaustive as I haven't fully followed through on checking file dependencies etc.
Note also that the pin assignments used in the code use Broadcom GPIO numbers (BCM). These are the GPIOxx numbers on the diagram below, not the actual pin numbers on the header:
AlphaBot2.py appears to control the robot's motors. The pin assignments are:
Ultrasonic_Obstacle_Avoidance.py and Ultrasonic_Ranging.py specify:
TRIG = 22
ECHO = 27
Which are obviously the pins allocated to your ultrasonic sensor
Similarly, Infrared_Obstacle_Avoidance.py specifies
DR = 16
DL = 19
which are the pins used by the IR sensor
CS = 5
Clock = 25
Address = 24
DataOut = 23
Button = 7
IR = 17
For your remote control
Joystick.py assigns the following pins:
CTR = 7
A = 8
B = 9
C = 10
D = 11
BUZ = 4
Line_Follow.py also sets GPIO7:
Button = 7
Finally, ws2812.py sets the LED pin to 18
LED_PIN = 18 # GPIO pin connected to the pixels (must support PWM!).