Bittle X+Arm

A servo plug has three wires. Plug it in the right direction, or you may burn the chip. The color of the wires may vary across models. However, the darkest-colored wire (Black or Brown) is always the GND (ground) wire by 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.