Editing an existing robot

[jumpjack]

I forked a repository of a robot arm, to try editing it to meet me needs:

https://github.com/jumpjack/robot-arm-3d-with-physics

But as of now I was only able to change colors of the pieces.... I don't undrestand how/where each piece is positioned and rotated, and how joints definition work.

Robot is defined here.

This part define the shapes, and it's ok for me:

...
        this.base = utils.addBody("position", "cuboid", world, scene, 1, 100, -1, r_d, 0.4, 0.15, 0.4);
        this.mast = utils.addBody("dynamic", "cuboid", world, this.base.m, 1, 100, -1, r_d, 0.075, 1.5, 0.075, 0, 0, 0, 0xff00ff);
        this.indicator = utils.addBody("dynamic", "cuboid", world, this.base.m, 0, 0, -1, r_d, 0.02, 0.02, 0.02, 0, 0, 0, 0x0000ff);
...

But about the positioning, I see that only grip parts have a position.set() statement, so how/where are the other parts positioned and rotated?

Then the joints: currently the arm rotates around the mast:

Instead I want the upperarm to rotate around its own axis when I move the grip:

How can I do it?