How does it work ?

The video is not very helpful and it has lots of features the 0.92 version doesnt have.
https://www.youtube.com/watch?v=m1sj4AeU3Yo

So for example:
i draw a shape with the Bezier Tool, apply the LPE and set the mode to Vertical Page Center.
That mirrors along the center of the page.

How do i add more shapes to the existing path, so it automatically gets mirrored ??

Path>Combine works for me, when the mirrored path is higher in the z-order.

I was thinking that grouping the first shape, then applying the LPE, and adding more stuff inside the group should automatically add, but that isnt the case....

Mmh. It works for me - but the object needs to be an actual path. It doesn't work when I add a star to the group. But as soon as I convert to path, it works.

Windows 7 Pro, 64-bit, Inkscape 0.92, 64-bit. I haven't seen the video, and this is just trial and error.

Once I apply the Mirror Symmetry, the original path and the mirrored path are automatically a single object. Not grouped, but a single object. So one can't be higher or lower in z-order. If I add a new path and Combine with the original + mirror, the new path is added to the mirror object too. It doesn't matter whether the new path is higher or lower in z-order, it works either way. (for me)

I tried the grouping trick in 2 different ways. First, making a group after applying mirror symmetry, adding a new path to the group does not result in a new path in the mirror object. The group is the original object, the mirror object and the new path. But if I make the original object a group before applying the LPE, and I add a new path to the group, it does add the new path to the mirror object.

At first, I thought it wasn't working properly, because I had entered the group to paste in the new path. I'm always forgetting whether I'm still inside a group. But once I came out of the group, I saw it had worked properly.

If I extend the path using Pen or Pencil tool, it extends the mirror object as expected.