Bittle X+Arm

A servo plug has three wires. Plug it in the right direction or you may burn the chip. The color of wires may vary for different models. However, the darkest-colored (Black or Brown) wire (GND for ground) is always the GND wire as a convention. The GND wire should be plugged closest to the board.

The index number of the joint servo has no corresponding relationship with the PWM pin number on the main board. You don't even need to read the pin numbers on the PCB board.